An age-old rule of the internet, from Usenet, to AIM, to facebook, to anything you use today - ignore shit you don't like. Engagement feeds assholes, narcists, trolls, and crazy people. Ignoring them (beyond acknowledging their existence and moving on) starves them of engagement. Either they move on to easier sources of engagement or they'll change their behavior. It's a like child who thinks there's no real difference between punishment or praise because they're both kinds of attention and that's what the child really wants. Denying them the attention they crave is the only way to get them to change and seek positive engagement. The only alternative is banning someone from a forum, which I generally disagree with on principle except for egregious situation.
You can't control other people's behavior, but you can control how you react to them. Put them on ignore until they learn to behave in acceptable manner.