I've always thought that a simple "ignore all posts/threads" function (where each member can choose to hide the posts/threads by those posters that they don't wish to view any more) would work at discouraging trolls. No need to have a 'community' value asserted, everyone is free to decide who is/isn't a troll, and set their own personal filter on them. If enough people think the same, the troll will end up shouting into the ether, with no views or responses.