A griefer, in computer games, is a player harassing other players.
On a general way, griefer abuse of the game mechanism, preventing other to counter, in the purpose to cause grief.
On SWRP, the sense differs from a sim to another. For example, the use of OOC chat or devices may be allowed on place where roleplay is not enforced. Also killing players without a mutual roleplay may be allowed in some place a intended realism, while not in some others.
The victims can defend, moving away, ignoring the griefer or complaining to the sim staff. Replying by similar acts is not recommended.