Depends on how I'm feeling, really. Sometimes I'm feeling verbose, so I like to use them as an argument board. Just spew out ideas, see which ones stick. It can be fun, depending on the situation.
Most of the time, I just ignore them, however. No sense in getting riled up about something someone wants you to get riled up about.
That is something I don't understand about watching people debate, though. Your opponent insults you to throw you off your game. Why get mad when that's exactly what they want you to do? Keep your cool, insult them back, and you'll win pretty easy. Getting mad at a troll is like beating a masochist. It's what they want, don't do it unless you get something out of it, too.