Has there ever been a massacre or genocidal act via people thrown off a cliff?

I don't know about cliff genocides - a very large cliff or a very small tribe would be necessary for a cliff genocide.

But cliff massacres are another story. If one agrees on the minimum number of persons for a massacre, one can find a few cliff massacres.

American Indians sometimes drove buffalo herds off cliffs, but I suppose that buffalo don't count for massacres.

I think that in the movies Solomon and Sheba (1959), and Taras Bulba (1962) feature battle tactics causing enemies to fall off cliffs, Trooper Hook (1957) begins with a cliff massacre, and Smoke signal (1955) has a one man cliff massacre. But those are fictional movies.

Many historical persons died from falls, deliberate or accidental.

Jan Masaryk (1886-1948) foreign minister of Czechoslovakia, jumped, fell, or was pushed to his death 10 March 1948.

On May 23, 1618, a crowd of Protestant leaders defenestrated two of the regents of Bohemia and their secretary out of a 70 foot high third story window in the Bohemian Chancery at Prague. They survived, but this led to the Thirty Years War.

On 30 July 1419, a pro Hussite crowd defenestrated the judge, the burgomaster, and 5 city council members from the New Town Hall in Prague. All seven died, and the Hussite Wars resulted.

https://en.wikipedia.org/wiki/Defenestrations_of_Prague

In 1195 Alexios Angelus deposed, blinded, and imprisoned his brother Isaac II and became emperor Alexios III. Isaac II's son Alexios (c. 1182-1204) escaped from prison in 1201. In 1203 the Fourth Crusade attacked Constantinople, and Alexios III fled. Isaac Ii was restored to power and Alexios IV made his co emperor, but they were unable to pay the crusaders all that Alexios IV had promised. Alexios Doukas overthrew and murdered Alexios IV and Isaac II in January 1204 and became Emperor Alexios V.

The Crusaders captured Constantinople on 12 April 1204 and formed the so called "Latin Empire". In exile, Alexios III married his daughter to Alexios V, but later blinded him. Alexios IV was captured by the crusaders and executed in December 1204 by being thrown off the top of the Column of Theodosius.

Thee is a story about Henri Christophe, King of Haiti from 1811-1820, receiving a foreign envoy at his cliff top Citadelle Laferriere. According to the story he demonstrated the iron discipline of his troops by ordering a unit to march across the parade ground. When the troops reached the battlements over the cliffs they received no orders to stop, and so continued marching over the battlements and fell to their deaths.

If true, that story would be the best example of a "cliff massacre" I can think of.

One biography of Cervantes (he was prisoner in N. Africa, ~ 1575-80) tells the following:

In N. Africa (Algiers, I think) there were a high cliff from where slaves or unransomed prisoners were thrown to their deaths. In the cliff walls there were various kinds of large hooks and spikes embedded in the rock.

It was the masters' sport to bet on how and how quickly the slaves/prisoners would die. Some did not hit any hook and died instantly when hitting the stones in the sea floor; Some would be lacerated by some hook or metal spike and be torn in pieces during the fall; Some would get stuck in the hooks, sometimes taking hours to die, depending on how much they were hurt or bled. Some would be stuck in the hooks by their chains, being not really hurt, and thus would die by dehydration in a few days. Lots of variations to bet on!

'unransomed prisoners' means victims of Muslim piracy in the Mediterranean or Atlantic, who were not able to be ransomed or valuable as slaves (families or charitable orders would often pay their captors for their release, most famously the Mercedarians )

And it look like throwing them off a cliff is one of the 5 accepted ways to execute gays in Islamic jurisprudence, at least accordingly to Shaykh Hamza Sodagar and others. So the ISIS habit of throwing them from high buildings has precedents, after all. One Tunisian iman cites a hadith directly from Mohamed (see at 4:00): The Prophet Muhammad said: “Whoever you find engaged in sodomy – kill both the man who does it and the man to whom it is done.” But the same Tunisian also clarifies that throwing from a high place is the preferred method only for the Hanafi school, others prefer other methods.

7-800 soldiers were forced or pushed off of a 1000 foot cliff in one of the unification battles of Hawai'i. Wikipedia article

This was one of the last major battles of the campaign. The article details how the remains were discovered during construction 100 years later (in 1898).

Yes, in the Awa'uq Massacre, or Massacre of Refuge Rock.

In this episode Russians under fur magnate Grigorii Shelikhov fired on Koniag Alutiiq people (per Wikipedia, those of the Qik’rtarmiut Sugpiat tribe) massed atop the large rock. One source, Briutikov, is reported to have said that 500 were killed in their fall from the rock. Owen Matthews in his book "Glorious Misadventures" (p. 65) describes the account of another witness, Izmailov, who said that "probably many more" victims jumped or fell from the rock than were killed there.