Massively has a writeup discussing the way CCP Games is battling ISK-farmers in EVE Online
(ISK is the game's currency). The developers felt that merely banning sellers whenever they could was not enough, so they introduced a system where players could purchase game-time codes that could then be sold within the game
to other players. Since players are unlikely to give up buying ISK voluntarily, CCP's thought is that they can at least keep the money and currency distributed among the real players
. Some of the player-base has been critical of the plan, but it's becoming more and more popular as time goes on — and the old ISK-sellers aren't pleased.