What Does Griefer Mean?
A griefer is a player in a multiplayer video game that goes out of his or her way to annoy other players. The term griefer is derived from the idea of “giving [someone] grief”. There are many, many ways that a griefer may go about angering others, including:
Techopedia Explains Griefer
Much like chat room users can report trolls, gamers can report griefers. Once a person is accused of griefing, they are given warnings and other due process before being removed from the game if they do not change their behavior. It is a difficult balancing act for companies because they want to encourage competition, but not abuse.