A user of my mod pointed out that you can attacking someone, use a calm spell, pay off the assault charge (which is generally cheap), let the victim attack you when the spell wears off, and then murder said victim without incurring a murder fine. So, I was thinking that perhaps there'd be a way for a script to stop everyone from attacking you when it gets paid off. Any thoughts?

I dont know, I would still be pissed if someone slapped me. This is an interesting loophole. I always thought it would be nice if they were alone no one would know anyway or they would run and get help because the auto knowing thing is kinda unrealistic.

If I'm not mistaken, "player"->StopCombat stops all NPCs from attacking the player, since the command is supposed to stop combat for all involved in a fight. You could try adding that to the results box of the guard's 'fine paid' dialog. Although I doubt this will work for NPCs who are not in the active cell(s).