This guy won't leave me alone, how do I get him to go away?

There's this guy who I barely know who keeps sending me facebook messages. He's in my science class and we'd never even talked before he started sending these messages a few months ago. At first he was okay, just trying to have a nice conversation, but then he asked for my number. I didn't want to give it to him but I did anyway just to be polite. Now he texts and messages me pretty much non-stop, stares at me in lessons and bugs me to meet up with him all the time. The other day he was sending me messages and he said he'd noticed I was wearing a new skirt to school and that it was 'sexy' and he started making remarks about my ***. I tried to change the subject and let him know that I didn't like him saying that, but he didn't get it and just carried on. Yesterday I said I would meet up with him because I couldn't think of any more excuses, and when we met he kept trying to put his arm around me and hug me and said some stuff that I didn't like the sound of. I ended up making some bad excuse about having to leave, and I thought maybe he'd get the idea that I'm not interested, but apparently he hasn't got the idea yet. I always feel really weird around him, and just now he's sent me a marriage request on facebook without even asking me before, even though I'm already 'married' to someone on there. I guess I wouldn't mind just being friends but it feels like he's trying to push stuff on me and it's getting kind of creepy and I feel really uncomfortable. If I don't reply to his messages within a few minutes he just sends more and it's really annoying. I don't want to tell him outright because I don't want to seem rude...how do I let him know I'm seriously not interested and get him to leave me alone?