Grief

Usually to PK repeatedly, but has other definitions. A griefer in World of Warcraft is almost always one who PKs or trains mobs to other characters, but the term includes any activity specifically designed to hamper another player's gameplay, progress, or fun.

Additional notes:
 * On servers designated as "PvP," activities such as corpse camping and ganking are not considered griefing by Blizzard and will not be punished by Game Masters.
 * Griefing can also be harassment as it is defined in the EULA, which can result in disciplinary action for a player.

Also see Gank and Griefing.