How to address wrong view

"From what arise contentions and disputes, lamentations and sorrows, along with selfishness and conceit, and arrogance along with slander? From where do these various things arise? Come tell me this."

"From being too endeared (to objects and persons) arise contentions and disputes, lamentations and sorrows along with avarice, selfishness and conceit, arrogance and slander. Contentions and disputes are linked with selfishness, and slander is born of contention." - Kalahavivada Sutta (Sn 862-877).

In the Mahayana tradition the last Bodhisattva branch vow (#16) reads

(16) not correcting others who are motivated by delusions When we see somebody is making a mistake, we must talk to them with the right motivation and try to help them correct that mistake. If we help other people not to make a mistake again, that is very helpful for that other person but it will also indirectly help us as well.

The proper way to address this is also highly situational.

Keep in mind some of the other bodhisattva vows that must be kept in tandem, especially

(27) abandoning the path of Theravada as unnecessary for one following the Mahayana

(32) praising oneself and belittling others because of pride and anger

4. (root not branch)

doubting and denying the doctrine of the Great Vehicle

Also

7. causing an ordained person to disrobe

(34) disparaging a spiritual master

and

(45) not acting with whatever means are necessary according to the circumstances to stop someone who is doing harmful action

So in summary one should be certain of the following

Do you have the full context?

If you walked in to a room and heard a sentence that might be out of context you might want to hold off on accusing someone of spreading the wrong view

Are they talking about a Buddhist tradition other than your own?

The vows are very specific in stating that to disparage another tradition, or to cause rifts in the dharma is a downfall.

Are you reacting out of anger or pride?

If so it is better to withdraw, perhaps discuss with the person later when you have leveled your emotions.

Are they a guru or monk?

If they disrobe and give up on their quest it is a huge downfall, is this a patterned behavior? Is this perhaps a lesson tailored to a specific individual? Be certain before going on the route of accusation.

And finally, say you are certain beyond a doubt they are in the wrong, say for this example they claimed that "The Buddha says kill all third born children" an obvious wrong view and greatly harmful.

Then you would be obligated by the vow, to do something... If you have taken the vows.

As stated before it is highly situational.

(10) complying with the minor precepts when the situation demands one's disregard of them for the better benefit of others

This last branch vow is an interesting one, one skillful means described by my old guru back when I was practicing the vows and still considered myself Buddhist was the act of reprimanding with false anger. To appear angry and scold when you actual are not holding anger and are holding compassion for the one being scolded.

If someone was a guru or a monk and they were telling people to kill 3rd born children (A highly unlikely case) It is permissible to break certain vows to stop them and their followers.

See

(45) not acting with whatever means are necessary according to the circumstances to stop someone who is doing harmful action

The last two downfalls are saying that, although we must not do anything to harm sentient beings, there are circumstances where we might have to act quite forcefully to stop them harming themselves. When a person or a group is causing themselves or others pain and difficulties, really unlawfully or immorally treating great numbers of sentient beings badly, we should not just accept that, saying we are Buddhists and therefore passive people. We should oppose them skilfully.

Extract from Mahaparinibbana sutta

Then the Gracious One together with a great Community of monks arrived at Bhoganagara. There the Gracious One lived near Bhoganagara at the Joyous Shrine.

There the Gracious One addressed the monks, saying: “I will teach these Four Great Referrals, monks, listen to it, apply your minds well, and I will speak.”

“Very well, reverend Sir,” those monks replied to the Gracious One, and the Gracious One said this:

1) “Here, monks, a monk might speak like this: ‘I have heard this directly from the Gracious One, friends, directly I learned it: “This is the Teaching, this is the Discipline, this is the Teacher’s Dispensation.”’ That monk’s speech, monks, is not to be rejoiced over, not to be scorned at. Without having rejoiced over it, without having scorned it, after learning those words and syllables well, they should be laid alongside the Discourses, they should be compared with the Discipline.

2) If, when these are laid alongside the Discourses, compared with the Discipline, they do not fit in with the Discourses, they do not compare well with the Discipline, you may here come to this conclusion: ‘Certainly this is not the Gracious One’s word, it is not well learned by that monk,’ and, monks, you should abandon it. If when these are laid alongside the Discourses, compared with the Discipline, they do fit in with the Discourses, they do compare well with the Discipline, you may come to this conclusion: ‘Certainly this is the Gracious One’s word, it is well-learned by that monk.’ This, monks, is the first Great Referral you should bear in mind.

3) Here, monks, a monk might speak like this: ‘In a certain dwelling place lives a Community with elders and leaders, I have heard this directly from that Community, directly I learned it: “This is the Teaching, this is the Discipline, this is the Teacher’s Dispensation.”’ Those monks’ speech, monks, is not to be rejoiced over, not to be scorned at. Without having rejoiced over it, without having scorned it, after learning those words and syllables well, they should be laid alongside the Discourses, they should be compared with the Discipline.

4) If, when these are laid alongside the Discourses, compared with the Discipline, they do not fit in with the Discourses, they do not compare well with the Discipline, you may here come to this conclusion: ‘Certainly this is not the Gracious One’s word, it is not well learned by that Community,’ and, monks, you should abandon it. If when these are laid alongside the Discourses, compared with the Discipline, they do fit in with the Discourses, they do compare well with the Discipline, you may here come to this conclusion: ‘Certainly this is the Gracious One’s word, it is well-learned by that Community.’ This, monks, is the second Great Referral you should bear in mind.

5) Here, monks, a monk might speak like this: ‘In a certain dwelling place live many elders, very learned, who have learned the traditions, who are bearers of the Teaching, bearers of the Discipline, bearers of the Tabulation, I have heard this directly from those elders, directly I learned it: “This is the Teaching, this is the Discipline, this is the Teacher’s Dispensation.”’ Those monks’ speech, monks, is not to be rejoiced over, not to be scorned at. Without having rejoiced over it, without having scorned it, after learning those words and syllables well, they should be laid alongside the Discourses, they should be compared with the Discipline.

6) If, when these are laid alongside the Discourses, compared with the Discipline, they do not fit in with the Discourses, they do not compare well with the Discipline, you may here come to this conclusion: ‘Certainly this is not the Gracious One’s word, it is not well learned by those elders,’ and, monks, you should abandon it. If when these are laid alongside the Discourses, compared with the Discipline they do fit in with the Discourses, they do compare well with the Discipline, you may here come to this conclusion: ‘Certainly this is the Gracious One’s word, it is well-learned by those elders.’ This, monks, is the third Great Referral you should bear in mind.

7) Here, monks, a monk might speak like this: ‘In a certain dwelling place lives one elder, very learned, who has learned the traditions, a bearer of the Teaching, a bearer of the Discipline, a bearer of the Tabulation, I have heard this directly from that elder, directly I learned it: “This is the Teaching, this is the Discipline, this is the Teacher’s Dispensation.”’ That monk’s speech, monks, is not to be rejoiced over, not to be scorned at. Without having rejoiced over it, without having scorned it, after learning those words and syllables well, they should be laid alongside the Discourses, they should be compared with the Discipline.

8) If, when these are laid alongside the Discourses, compared with the Discipline they do not fit in with the Discourses, they do not compare well with the Discipline, you may here come to this conclusion: ‘Certainly this is not the Gracious One’s word, it is not well learned by that elder,’ and, monks, you should abandon it. If when these are laid alongside the Discourses, compared with the Discipline, they do fit in with the Discourses, they do compare well with the Discipline, you may here come to this conclusion: ‘Certainly this is the Gracious One’s word, it is well-learned by that elder.’ This, monks, is the fourth Great Referral you should bear in mind. These, monks, are the Four Great Referrals you should bear in mind.”

In the case of words that the Tathagata knows to be unfactual, untrue, unbeneficial (or: not connected with the goal), unendearing & disagreeable to others, he does not say them.
_{--MN 58, Prince Abhaya Sutra}

In this vein, I think you can address the problem by showing how "someone's expounding their beliefs in a public forum" is "unfactual, untrue, unbeneficial (or: not connected with the goal), unendearing & disagreeable to others"

However,

There are here, O monks, some foolish men who study the Teaching; having studied it, they do not wisely examine the purpose of those teachings. To those who do not wisely examine the purpose, these teachings will not yield insight.
_{--MN 22, The Snake Simile Sutra}

Based on the above I feel that by shifting our focus from the metaphysics ("how things are") and towards the purpose (i.e. soteriological methodology -- "how to act towards Bodhi") we can all avoid hurtful and non-productive arguments.

Of course others have responsibility to rectify the wrong views. There were several individuals spreading wrongly grasped views on Dhamma even during Buddha's time like Arittha, Sati and Sunakkhattha. Buddha's and other monks' reactions on such situations are detailed in Alagaddupama sutta and Mahatanhasankhaya Sutta.

Buddha explains the plight of such people, as if grasping a snake from it's coils in the water snake simile in Alagaddupama sutta.

there is the case where some worthless men study the Dhamma... Having studied the Dhamma, they don't ascertain the meaning of those Dhammas with their discernment. Not having ascertained the meaning of those Dhammas with their discernment, they don't come to an agreement through pondering. They study the Dhamma both for attacking others and for defending themselves in debate. They don't reach the goal for which [people] study the Dhamma. Their wrong grasp of those Dhammas will lead to their long-term harm & suffering. Why is that? Because of the wrong-graspedness of the Dhammas.
Alagaddupama sutta

But for people studying the Dhamma for its real purpose, that is the cessation of suffering, Buddha's advice is to question him or an experienced monk when there's doubt.

"Therefore, monks, when you understand the meaning of any statement of mine, that is how you should remember it. But when you don't understand the meaning of any statement of mine, then right there you should cross-question me or the experienced monks.
Alagaddupama sutta