I like the idea with the thumb move.
TICKLE TICKLE TICKLE
*thumb move!*
Boyfriend: Owwww! What the hell?!
Girlfriend: I told you to stop. Let me know if you want another.
Do this once or twice, and he'll either:
A) Hit her, at which point she calls the police.
or more likely B) he will stop doing it.
Still, it'd be nice if he just got the point, instead of having to cause physical pain. But hey -- whatever works for dogs will work for men. Ducks are a bit more sophisticated.