Downvoting moves the comment down the reply tree and greys it out making it harder to read. Flagging comments completely hides them from many users, especially when logged in. And flagging submissions sends them to oblivion.

So these methods are definitely used to suppress topics or opinions, for better or worse. But when it comes to genocide it's obvious that those committing it also have the power to abuse every mechanism available to suppress information and discussions condemning it.