A griefer is a player who plays a computer game simply to aggravate and harass other players, causing grief, instead of playing the game in order to achieve objectives defined by the game world and having fun with others. Their fun is in hurting or angering other players. It can be done with exploiting unintended game mechanics, but the main purpose is the negative impact to the gameplay of another person. (For more see Wikipedia.)
This attitude is strictly against the spirit of the Rules, and any instance of poor gamesmanship should be reported to the DMs.