I mean, what is it? You make inflammatory statements you don't even believe in, watch people who would get offended rage at your lies for a while, then tell everyone you were trolling, then trololol at their reactions. What purpose does it serve? Even if the ragers were a non-protected group like fans of something, you purposely offending them for the lulz is destructive to community spirit and should be cause for websites to ban you.
I don't mind, they're like a punching bag. You can yell at strangers over the internet which actually I find relaxing. It's like you can release your anger at someone who's purposely being ignorant, and afterword you just feel a bit more calm about the real people around you. Yay for trolls, the punching bags of the internet!
How do you pick a Troll, since some consider belligerent, strongly opinionated individuals as trolls and may complain and have them wrongly booted as trolls, or some people are on DDO to push a stupid opinion or to try and convert people over to their stupid religious beliefs or proselytizing, which makes them appear like trolls or just stupid.
These would likely be kicked from complaints against them as trolls, while the real trolls are clever and careful as to how they conduct their trolling and so often don't get picked as trolls. Though there is the occasional, unintelligent troll who is absolutely obvious, but they usually get ignored anyway. Nobody likes feeding the trolls and they usually leave or mess up badly an make defamatory, disgusting and derogatory remarks that get them expelled anyway for poor conduct.
People rarely even ever read the terms of service, especially not trolls. Besides, how exactly would they define trolling, or attempt to punish those who are trolling, and thus breaking the terms or service? Trolling can take many forms. Also, a well planned, and executed troll can actually be quite hilarious, and in quite a few cases, the one being trolled kinda had it coming. Then there is also friendly trolling, the good natures teasing and messing with people that all buddies do. Why should that be against the terms of service.
A better idea would be to ban bullying in the terms of service.
First off: Who reads the terms of service? Almost nobody. Pure rolls are much less likely to read the terms of service, and certainly less likely to follow them, which negates any preventative power.
Second off: If you need a justification to ban them, "inflammatory content" or similar would fit under the vast majority of website's terms of service, which would have the same effect without needing to mention trolling specifically,
Trolling online comes in many shapes, forms and ways. From obvious, like insulting people, to more subtle stuff like convincing someone to get themselves banned or secretly getting people to click a rick roll link or something. The fact is there is no definite definition of Racism, just like no definite definition of "What".
And frankly, trolling isn't always bad. It does add comedy to forums when needed.