Read the historical definition of the word "troll", as in a monster that lives under a bridge. Don't let a small minority of people dominate the whole forum, and possibly run better potential members off. Low-class people have a tendency to find each other, form strong cliques, dumb the discourse down to their level and anyone that isn't immediately comfortable rubbing elbows with a pack of low-end degenerate drunks, drug-addicts and ne'er-do-wells will quickly migrate somewhere else. I watched a bar owner inform a group of "regulars" that he didn't want them to come around any more. They'd camp at one end of the bar, every night, night after night, pressure anyone that was in "their" spot to move somewhere else, didn't spend much money, dominated the whole room, and the bartender's time, were loud, stupid, obnoxious and acted like they owned the place. Until the owner threw them out for ever. Told them to go up the street, to another place, that didn't care about it's image, or the quality of it's customers.
Most forum owners assume that as long as people aren't using bad language, and aren't "offending" anyone, then everything is good. Not true. You can have a well-populated forum filled with idiots, who have nothing to offer a newcomer that is not an idiot. I see there are a lot of well-behaved members here, that don't use bad language, and don't offend anyone.