I usually try to ignore most trolls, but unfortunately many of those I encounter tend to be rather prolific. The occassional silly nonsense is tolerable (or occassionally funny), but prolonged exposure to stupidity-fueled arrogance will get to me. Depending on the situation, I may or may not intervene by pointing out all the faulty logic the troll has been using; since the cases which get to me tend to be chronic, that is often pointed out as well.
However, I'm usually trying to not incite the trolls myself (I just lack the malice needed to do that, regardless of if they do deserve it or not). Rather, I'm simply trying to get them to back them into a metaphorical corner. Most of the time, I aim to do this by denying this adage:
"Never argue with an idiot. They'll bring the conversation down their level and win through experience."
As noted before, trolls are seldom known for being knowledgable. By constantly forcing the debate into more intellectual arena, the troll can't continue to act as they normally would without making it clear that they are an idiot; the only other responses are to either continue the discussion in it's more eloquent state or simply stop altogether. It's very rare for a troll to step up their own game at this point, but at that point it becomes a battle of wits (hopefully you're ready for this one, because this can make things backfire on you spectacularly) rather than an attempt to piss off everyone on a forum.
However, one should be careful while using this approach to not fall into the trap of using ad hominem. The intent is to not troll the trolls, just to stop them from trolling by making the discussion a more intelligent one. This requires you to call them out on their bad behaviour, but you can't stop there without committing the fallacy yourself. As odd as seem, the solution to this problem is to help the trolls; give them advice for how to be more intelligent in their discussion, state which (if any) statements they have made are valid, and so on. If they don't heed your advice, it will demonstrate that the troll doesn't know what they're talking about.
Anyhow, that's how I deal with trolls.
