What if they accidentally shoot a troll thief they thought was unprotected, or accidentally lag on a brahmin gate, and become Antipathy? The game can't tell the difference between just shooting someone and shooting a troll that was trying to trick you.
It has been suggested, but it's still a very good idea. If some form could be implemented. I know that alts will still arise. A possible limit to characters is an option, not a great one but still. In whatever form, it should also extend to thieves and door blockers. (I know they do get neg rep but it should last as long as any other rep changes do.)