I have two suggestions for when people start roleplaying out of hand. First, drop that person a tell. Something along the lines of "Yes, PS is about roleplaying, but there are rules too," except a little longer and more precise to make it crystal clear what you're talking about. If the person continues to be a pest, send a petition. I can't speak for other GMs, but I will definitely intervene if I feel someone is disrupting roleplay. Probably, I'd do something like this: /tell to explain the issue (similar to what the player should do above); warning message if I get no response; if the problem persists, one last warning in a /tell with a threat to mute; /mute.
A lot of players feel that if someone is messing up roleplay, they have to go along with it in-character and try to make the person out to be a lunatic or find some other way of making sense of it. This is not the case. If someone is acting out-of-character, you don't need to incorporate that person's actions into your roleplay at all (you might send a /tell to other participants in the roleplay suggesting that everyone ignore the person). That person is in a different world, invisible to your character. Don't worry about out-of-character players; just do your thing and call for a GM if they get out of hand. Again, I can't speak for everyone, but I'll certainly do my best to help with that sort of thing when I'm on.
