> Any evidence of a language being stamped out by a despot?

Brazil had a couple of instances of that. In the 18th century, the Marquis of Pombal implemented a program to extinguish usage of Tupi as the national language and replace it with Portuguese. That was successful. And then a second iteration when the Italian, German and Japanese communities of speakers in South Brazil were prevented from speaking their languages around the time of World War 2. This was successful too.

Police document declaring it illegal to speak German, Italian and Japanese in public:

https://en.wikipedia.org/wiki/File:Brazil_-_Proibido_falar_a...