What's the acceptable, mature reaction to witnessing a stranger smack your GF's butt?

We've all seen the memes "this guy smacks your girl's ass, wyd?"

Seriously, what are you supposed to do in this situation? Obviously violence is never the answer. Do you call the police? Confront the perpetrator? Immediately leave the scene and console your gf?

Depends on the situation. In a bar or party? Tell him that she's with you and to leave her alone.
Alone in an alleyway or something? Get the fuck out of there and call the cops or something.

let the person know she's not alone and that doing that is harrasment

I think brushing it off is not acceptable, but not sure how far to go in response is appropriate. It is sexual harassment after all.