>>sharps+(OP)
My experience on forums is that -size matters-. A pond with ten fish gets far less trollers than a pond with ten thousand.
As for solutions: there have to be consequences. Either losing the ability to post or losing the ability to have your posts seen. Certainly banning accounts is an option, but then you have to fight with the crapflood, if and when it comes.
>>boredg+w
I think the community size bit is important (see 'monkey number'). I also wonder if that means that sites should somehow attempt to break people into smaller groups in some way in order to maintain that 'small town feel'.