OCG, what should people do, when encountering people who advocate violence against others?

There are people who are that hateful in this world, and they support things such as violence, murders, or exterminations against other people.

Sometimes you might find some on the Internet in a chance before the mods delete their posts, and sometimes they can be seen in real life. Examples include some Internet trolls, politicians, some supremacists (Black, White, etc.), and terrorists (like ISIS).

So, if anyone were to encounter those people, who may threaten or support attacks against other people, what would you like them to do?

Should they support, oppose, or ignore those advocates?