Make it so when someone presses esc or attempts to leave the game it gives them a loud warning sound and popup saying "Leaving during combat" You are about to leave during combat, you will lose XP, 10% of your bank balance (and whatever else it is). Are you sure you wish to proceed? You have (time remaining) time remaining.

The amount of times I've left during combat on accident because I didn't notice i was still in combat is crazy, if it had this it'd prevent that a lot