When a rogue in your party is trying to sap (or near a mob while in stealth), use the Halloween costume wand just before he does, it removes stealth, thereby he will die and get durability damage.
The good thing is that its hard to find out who did it, and if he does, only say that it was an accident, and you were just trying to give him a nice costume
EDIT: This can also be used in pvp to suddenly appear him in the middle of a horde/alliance group