There are various ways to do anything, depending on the situation.
Personal story here: when I was working on a Smaug mud, a friend asked, "Why do paladins get disarm? How honourable is it to knock away someone's weapon and fight an unarmed person?" That led me to write a trigger. The paladin-type king would take all and then try to disarm the player. If successful, the king would teleport out to a dump room, drop all, wait (I think it was 2 sec), heal himself, teleport back and take all again. The pause would be enough to stop the fight without actually being visible to the player if there's no teleport message. I didn't like the message about fighting someone who wasn't there but the rest worked well. The king would have the weapon because of that second take all, give it back to the player and suggest they stop fighting.
If it's not the fighting so much as the killing that you want to stop, have the mob keep repairing any damage and you can add the message you want to the trigger.
