What would happen if the Pope were to abuse his power?

First of all, it is important to understand the very strict conditions under which a Pope declares things ex cathedra. To begin with, he can only make such a declaration about doctrine.

Here is how the Catechism of the Catholic Church describes Papal infallibility:

891 "The Roman Pontiff, head of the college of bishops, enjoys this infallibility in virtue of his office, when, as supreme pastor and teacher of all the faithful—who confirms his brethren in the faith he proclaims by a definitive act a doctrine pertaining to faith or morals." [...] When the Church through its supreme Magisterium proposes a doctrine "for belief as being divinely revealed," and as the teaching of Christ, the definitions "must be adhered to with the obedience of faith." This infallibility extends as far as the deposit of divine Revelation itself. [The documents quoted are Lumen Gentium 25 and Dei Verbum 10 of the Second Vatican Council.]

Nothing that is strictly disciplinary would enter under that protection. Even in real praxis, Canon Law is never "forever;" it can be changed by the legislator (the pope) as needed. So, for example, requiring Catholics to wear colanders and to pray to the Flying Spaghetti Monster (however ridiculous and scandalous that is) cannot be the subject of an ex-cathedra pronouncement.

In this extremely unlikely scenario, there is a basic principle of the Catholic theory of law that would apply: since a law must be for the sake of the common good, an unjust law does not bind in conscience. St. Thomas Aquinas, reiterating a principle already elucidated by St. Augustine, says,

[Unjust laws] are acts of violence rather than laws; because, as Augustine says, "a law that is not just, seems to be no law at all." [See De libero arbitrio, I, v, 11.] Wherefore such laws do not bind in conscience, except perhaps in order to avoid scandal or disturbance... (Summa theologiae, Ia-IIae, q. 96, a. 4, corpus).

A law can be unjust because it is excessively burdensome, or else because it commands people to act sinfully. Imposing the colander would clearly be excessively burdensome; requiring worship of the Flying Spaghetti Monster would be a kind of idolatry (or at best simply ridiculous and hence excessively burdensome).

So no, Catholics would not be bound to follow such decrees. Of course, if any Pontiff were to behave in this way, he would cause untold scandal and damage to the Church. Thankfully, no pope, not even the infamous Renaissance popes, or the (even worse) popes in the era following the fall of the Carolingian empire, has ever attempted anything of the kind. (They certainly abused their power in other ways, of course.)

Declaring the Creeds null and void would, however, enter into the domain of faith and morals. We have to keep in mind that—as the passage from the Catechism explains—when the Pope defines something ex cathedra, he has to

• Intend to act as the universal pastor. (I.e., it is not enough for him to give his personal opinion or even to act simply as the bishop of Rome.)
• Intend to proclaim whatever it is as to be held definitively by all the faithful.

It is the same intention and conditions that an ecumenical council has when it teaches.

In this case, Catholics believe that the Church as a whole (and therefore the Holy Father, by virtue of his office) is protected from teaching error. Nullifying the Nicene Creed, say, is clearly an error, and so the Church (and therefore the Pope) is prevented from performing that kind of action.

And in fact, no Pontiff has ever attempted such a reversal. (If you think about it, even the most disgraceful holders of the office would have had little motivation to do so.)

Supposing that the constitution of the Church were different, and that the Roman Pontiff were not protected from error, then, naturally, the faithful would not be bound to follow such erroneous decrees. (Such a scenario would, however, make it difficult to determine exactly which doctrines were the "correct" ones; the infallibility of the bishop of Rome, in the Catholic perspective, is an essential guarantee of the infallibility of the Church as a whole.)

(Another question, which perhaps the original poster had in mind is, "Could a pope privately be a heretic?" Could an closet Arian be elected the bishop of Rome? Could he confuse people by expressing his questionable or heretical personal opinions? The answer, at least in theory, is "yes." He would, however, be unable to impose his heresies "officially" as dogma.)

To answer the final question in the original post, in the case of a grossly misbehaved Pontiff, the only course of action would be to bring some kind of moral or political pressure to bear on him. Such a pope would be morally obligated to resign, but he would have to do so freely.