There are 2 types of internet trolls:
The old type, web 1.0 - they post on boards like here, usually controversial or silly topics. Or it could be an irrelevant response to a post e.g. ASCII art. They do it just to annoy the regular posters and mods, not to be taken personally. After a few repeat offences, they usually get banned from the board.
The new type, web 2.0 - they post in the comments section of deviantART, Facebook, Myspace etc of user pages. It's normally an attack on that user, so it is taken personally. They are difficult to ban because they have to be reported multiple times, before they are known to the mods. This is definitely the case with deviantART.
As for the OP, if it was death threats, then yes that's illegal. It would be the same if the death threat came by email, voicemail, phone call, SMS or good old fashioned letter.
If it's a flame war, personal insults, swearing, posting inappropriate pics, then I think that should be a matter for the mods, not the police.