Well I wouldn't feel too guilty about the smack now. It's done. I doubt you scarred him for life this once. Maybe it was enough of a suprise to discourage him next time.
Anyhoo, maybe make a more concentrated effort to not turn your back to him if you're unprepared (which is kind of a dumb thing to say, eh? LOL be prepared when you aren't prepared? :roll: )
Just anticipate it, and even back him up out of your space before you reach for the light. If he's crowding you, back him up. Seems to remind them to get out of your bubble. Just one tip, I'm sure there will be plenty more added to this.