Hmm... I've never had quite that situation arise, but I have had people come to (and in a couple of cases, join) my class who were attitudinal/behavior problems. My situation - teaching in a Y - is a little different, as there are plenty of people around. Here's what I've done, however: for the first occurence, students were requested to ignore the person (works great in most cases - but only if you can tell the students this when the idiot can't hear you - otherwise it become a challenge) OR have a senior student take the person aside or take the class so I can take the person aside, so s/he quits disrupting the class. In one memorable instance, when my assistant was out sick and only kids were present, I moved the class... which did a great job of alerting the Y that there was a problem, while preventing a direct confrontation. For repeated problems, or suspected repetitions (depends on circumstances) I have taken senior students (always adults) aside and given them the history of why the person is a problem or suspected problem, and enlisted their assistance, to either help contain the person or be responsible for getting assistance. I also have this system in place in general, for emergencies - if someone should be injured or suddenly become a problem; for example, I have several students with emotional disabilities, including a couple with anger management problems - it's never been a problem in class, but I have systems in place just in case.