You need to email him and say he needs to keep his crush separate from work. Make it very clear that you are flattered, but do not want things to be weird at work. Tell him he needs to keep things professional in the office. And if he continues to make you uncomfortable, talk to HR. If he's being abusive in the office or making your life difficult, it could be considered workplace harassment.
Otherwise, try to avoid him as much as possible. Don't attempt to go back to being friends. It's probably for the best that he unfriended you on Facebook. 24/7 attention from a coworker who you don't return feelings for is pretty awkward. He can go sulk all he wants. But once he brings it into the office, he's being inappropriate. Make it clear in an email that you want to be friends, but you cannot work in an unpleasant environment. You shouldn't have to dread going to work because this guy can't get over his crush. If he was sending you unwanted love letters at work, that's something he could get fired for. Tell him that you want to remain friends, but he has to cool it with the unpleasantness. If he still makes work difficult, let him know you're going to HR. That'll straighten him up right quick once he remembers all the love letters from him you have sitting in your inbox.
Otherwise, try to avoid him as much as possible. Don't attempt to go back to being friends. It's probably for the best that he unfriended you on Facebook. 24/7 attention from a coworker who you don't return feelings for is pretty awkward. He can go sulk all he wants. But once he brings it into the office, he's being inappropriate. Make it clear in an email that you want to be friends, but you cannot work in an unpleasant environment. You shouldn't have to dread going to work because this guy can't get over his crush. If he was sending you unwanted love letters at work, that's something he could get fired for. Tell him that you want to remain friends, but he has to cool it with the unpleasantness. If he still makes work difficult, let him know you're going to HR. That'll straighten him up right quick once he remembers all the love letters from him you have sitting in your inbox.
A friend of mine got fired because he asked out a co-worker three times. Not once did she say she didn't want to go out with him; she just said she had other plans each time - detailed plans. So the third time he asked she went to the boss who called in my friend, found out his side of the story, apologized because he understood where the confusion was, but fired him because he had to under the zero tolerance sexual harassment policy.
If a girl without enough spine to say, "I'm not that into you," can get a nice guy fired, I think you have a shot at making this guy understand that between sexual harassment and hostile working conditions he needs to get over it. If he doesn't, you need to talk to HR and/or your boss.
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