There's this acquaintance of both my girlfriend and I that pretends to flirt with my girlfriend. Neither of us are close enough with him to classify him as a "friend," though both my girlfriend and I get along well with him [except for this scenario] and run into him at several different social events as we have a lot of mutual friends.
Whenever both my girlfriend and I are together, he walks up to my girlfriend and "pretends" to flirt with her--he grabs her hair or makes jokes in a flirtatious manner. When this happens I usually step between then/whatever and kindly tell him not to do that (he is well aware that she is in a relationship). She has also directly talked to him. However, he keeps doing this. My girlfriend and I often talk about this situation and she has expressed dismay at the situation--she finds it very annoying.
He's mentioned to me before that he only does it because it annoys me [the boyfriend]. I've considered letting it be and "stop getting mad" so the perpetrator no longer gets a rise out of me. But, I feel this is an issue that should stop now, not something that should slowly fade away over a few weeks. Quite frankly, this behavior is unwelcome, is making us both uncomfortable, and quite clearly not acceptable.
He generally respects me, so I've considered sitting him down for a serious talk to tell him why it is not okay (he is several years younger than both of us), but I'm afraid of the small chance that it could make things worse. What's the best way (for either my girlfriend or I) to make it clear that this behavior is not OK?