I think if the guards in places like NCR made sure to loot ALL corpses, even loot from a player that got killed by another player. Make them attack anyone who tries to loot. This would stop the people who shoot you, get killed, and have a friend loot you. Call it payment for guarding the city.
edit: allow people to loot the corpse only if they are the one who killed him. So you can still kill thieves.