The major unfortunate factor in all of this, and the unbannability of a troll, is that like their DnD counterpart, a troll will try to regenerate when reprimanded for harassing the peaceful, good-natured locals. (Some even ignore the usual racial weakness of fire(y speech) and acid(ic retorts)) Trolls suffer little in the ways of deterrence. They will attempt, in all ways to return, especially if banned. Why is punishment ineffective to them?
~What they did cheesed someone off really bad; they won, even if they were defeated. You will forever remember the crap-stain on your gaming experience, they tainted a good memory of a good game.
~They glorify in this sort of recognition like a badge of honor. A banned troll is a proud troll, and a proud troll is only more fueled to spread his bad attitude.
~A proud troll also faces a personal challenge: returning to troll another day. They love this challenge, and even if they fail, they know they have a massive amount of other options.
~Trolls will try to convince you that you are just like them for lashing out at their antics. Self defense is not being a troll, (in my opinion, others -will- vary on this) an equally angry-sounding victim of trolling has every right to defend themselves, though they can sometimes be indistinguishable in attitude at this point, from the knob who threw the first punch.
~They paid for the game like the rest of us (And in no way do I endorse acting like a wad online just because you paid money.) Simply put, they feel that because they put the dollars down, that their money gives them the right to enjoy their online gaming experience in their particular way.
~If they can't return to troll another day in one game, they will move on to another. And another. And another. There are simply too many games out there, and that's a potpourri of opportunity for a lot of jerks.
The only real defense against a troll is a strong community and cooperation. Be vigilant. Document offenses both verbal and textual, and be persistent with the owners of your gaming community. You may dislike some of the people you play with, but chances are they've been victims too, and sharing your experiences will only add to a longer list of the slights and misdeeds made by the troll that can be presented.
Screenshot the crap out of things if it's text based in an MMO. Audio capture is great, but sadly, not an option, for gamers who want to get raw evidence of trolls in verbally abusive action. Collect infalliable evidence.
Persistence and making the entertainment provider realize they stand to lose several wallets by keeping one around, that sometimes speaks their language*. You just have to find the right way to infer it, that the troll in question both detracts from gaming experience AND revenue, so that you don't find yourself slapped with the soul-crushing response of 'Eh, we have tons of players, leave if you want.'* (Yes, it can happen.)
*(Not every gaming company is like this. Only one or two Multiplayer game companies stand out as abyssmal for customer support in regards to personal abuse.)
Most importantly:
Disregard, but do NOT ignore the troll. Turn the other way, and sure, mute them if it comes to that. But report them in whatever way you can as well. It's like most other community crimes, neighbourhood silence is begrudging acceptance and complacency. Deny the troll the brunt of your negative attention, and try your best not to flip out. (So hard, sometimes. So, so hard.) But on the other hand, build up their demise. Do it quietly, but with conviction. (If you brag that you're going to ban them, this will be their ultimate ecstacy, the greatest recognition as an ass that they can receive. )