The Guardian has produced an excellent column that addresses the problem of "griefers", people that abuse, team-kill and cheat other players of online games, particularly MMOs. Griefing has always been a problem where real players compete against their peers, although with massively multiplayer online games the problem has become much more serious. Earning a battleship in Eve Online or crafting a special item in World of Warcraft can take weeks if not months of game time. On top of that, the recent trend for people to sell items on auction sites like eBay has meant that rare online items have a monetary value.
The article cites several high profile situations where griefers have bent moral codes with the Guiding Hand Social Club's ruination of Ubiqua Seraph in Eve Online and the funeral crash in WoW being the two main examples. Solutions that companies have come up with to limit these types of situations from happening are also looked at with the Xbox Live points system and strong community measures (like guilds and friend lists) being top on the list of letting players self-govern the problem. Effective community governance is essential if companies want to keep players inside the game and so that the owners don't have to resort to more drastic measures, like Blizzard does with its periodic account banning binges.
Update: fixed a couple of minor grammatical errors.