I'm kind of playing devil's advocate here, but often I find the shooting of dead players happens because said player isn't making it clear they're hit and out of the game. For example at my last game I saw a group of players get hit and one of them still had his gun up and shouldered. One short burst later and he's bitching about being shot when already dead. Thinking about it he was probably just dicking about with his mates, but if you want to do that then don't do it in the middle of an active game area