I'd contact the non-emergent police line for your area as already stated and let them know what's going on. They'll probably have you come to the station or meet with an officer to get all the details and the officer will attempt to make contact with the suspect if they're local.

There's no need to retaliate or do childish things like post ads for gay sex...totally unnecessary and it's just going to create trouble. Make sure your girlfriend is safe in the meantime.

 

Yeah but its funny? Also if you know his address sign him up for some gay mags and bill me later. He will get that shit for like a year. Keep your girl safe and do go to the cops if you think he is serious if he is just being a douche then load him up with a ad and some mags he will like it!