Tough Situation

So I've been dating a girl for a while, since 2013, and we were on and off for a while, she was dating this guy who she didn't really like and didn't know how to dump. He was pressuring her into the idea to have sex with him, and he told me he would have sex with her at some point. Well, they eventually broke up and he moved away to the Midwest, I'm on the west coast and so is she, and me and her go to different schools. He's moving back next year to do his Junior year at her high school and I can't seem to stop him from coming here. He still wants to be with her. He's a seducer and a predator seeming guy but we have no proof that he is and no one would believe us enough to have something happen about it. My mother knows and says she doesn't think he's a threat but that she should be adamant about being safer when he moves. I want to help but I don't know how. This guy is the stuff my nightmares are made of even though it wasn't me he was pressuring. I literally had a nightmare about him last night, and I need help with this. I've blocked him on FB but I'm starting to wonder if that's such a good idea because I can't keep an eye on him. I've also blocked his # but I still have it memorized. I've tried going to school adults about this but they didn't do anything. Will be accepting of any advice. P.S. He's about 1-2 years older than us.