Tragedy of the Commons. What we really need is more incentive for people to maintain the quality of discussion, like if they were economically invested in the site.
See, for example, griefers on Medal of Warfare on Xbox Live.
They've paid for the game, and they're paying for the xbox live account, and there is reputation and reporting.
That does nothing to stop people like "General Minus" from trolling the heck out of other players.
Dedicated trolls / griefers will have many alternate accounts. Team AVO (Minecraft griefers) apparently had several thousand alt accounts. Or trolls will use other means. Grawp (wikipedia troll) posts disguised links that perform a wikipedia action. Lots of people click the links.
The other problem is that customers feel entitled. SuicideGirls, a NSFW site with a set of forums, had severe problems with awful posts. They implemented "spring cleaning" to sort it out, many customers expressed unhappiness, a few people were banned and a new board was introduced and low quality posting was tolerated in the other boards. A lot of drama for a tiny increase in quality on most boards.
