What if I want to kill someone to protect my allies (who are not involved in the fight; i.e miners in a field), but the guy logs before I can tell him not to come back. I either wait for two hours to ensure my allies' safety from this one guy, or risk their explodanation.
It should be a "No you can't come back" by default, with the option to allow them to come back if they ask. I really can't be assed to send PMs to every dude I kill to ensure my territory is kept clear (especially when I can't sit in front of the player list for two hours making sure no one comes back).