The closest we get to a modern day holocaust. Seeing how powerless anyone in the world is to stop it, it's horrifying to think what the Chinese government can get away with within their own borders.
You would have to convince at least the rich countries to stop trading with China and even then that could eventually lead to a war.
Realistically I think the US could start a soft war (blockade) with China (but they would have to be the one to start it) and try to starve it until it/the CCP collapses.