Dealing with a scumbag?

[That Guy]

My 'friend' is the biggest scumbag alive. About a year ago now, I told him some private stuff over Facebook, and he screenshotted it and put it on his wall, tagging me and the people I was talking about in the picture. That was over a year ago.

A few weeks ago, I'd half forgotten about the previous incident, and I was chatting to this person again, and he asked me a fairly personal question (I had told him I had a crush on a girl, but didn't tell him who, he wanted to know). I told him that I wouldn't answer it, I hadn't told anybody else, why should I tell him. He kept begging me and promised to keep it a secret, so I went ahead and told him that I liked this girl. Two minutes later, the same guy puts a screenshot of the conversation on Facebook, and tagged me and the girl in it. I managed to make him remove the picture before the girl saw it, but several other people liked it and commented on it. A few weeks later he took another screenshot of a page that I had liked (admittedly it was a soft porn page) and posted it on Facebook.

At school, he's not much better. If i'm with him and he sees the girl I like (or if he sees me and her together) he will almost always yell out something that embarrasses me or makes me look pathetic or whatever. He has also done this in the past with other girls. It seems if I have any chance of going out with a girl, he wants to destroy it. I have no idea why, does anyone have any ideas as to why he might be doing this?

But more to the point, what should I do about this guy? He's 17 and in his final year of high school (like me), so it's not like he's some immature 8th grader. I've blocked him on Facebook, but he still does the stuff with the girl at school. I want to get a little revenge, what's the best way of doing this?