I would go ahead and take him off your friends. Some sites don't alert the other person, so it might be a while before (if) he notices that he is not your friend anymore. And even if he does notice, so what? Unless you are actually trying to remain friends, there's no reason he should be your online friend either. Avoid him and contact with him as much as possible, in order to save yourself future pain. And sooner or later you'll wake up one day and realize you are over him!..

It's absolutely not childish to un-friend him. It's probably best, especially if he has such a strong effect on you, to cut him out of your life as completely as you can. If you happen to cross paths, just say hello and move on, literally. If he tries to chat, tell him you're late for an appointment and can't talk. Try to keep your contact with him to a minimum if it's impossible to cut him out altogether. And then, otherwise...time and living well will be the best way to get over him. It takes a long time sometimes but you can do it. Just be patient with yourself and treat yourself kindly, too...

If you're worried about appearing childish when you defriend him, is there a way you can block his updates and such so you're not reminded of him as much? Ah, just defriend him if you really need to get him out from under your skin. it's for your own good. You can always add him again when you're more over the situation. Good luck...

It's not childish at all do deleate him, in these cases you need to think about YOU and what makes you feel better. Good luck!..