How to punish player killers in online game? [closed]

There is a type of players in MMORPG called PK(player killers). They mostly kill other players for fun, to make everyone rage and basically trolling everyone. What would be the best way to punish them as game modder?

I thought about cutting their reputation, so NPCs won’t give them quests and not let them in cities. Another idea is bounty hunt.