Why is he treating me like this?

So I work with this guy who is a few years younger than me. When I first moved into the company he started asking me out etc. And I politely turned him down and said I was happy being single. Anyway cut to a few months later he kept sending me cryptic messages saying he had a crush and he kept flirting with me, I gently told him again that I like him as a friend and that I am seeing someone else (early days but I really like this other guy, A lot). Now he is being really cold towards me and quite nasty! The other night at work was the most awkward experience I have ever had, he completely ignored me, didn't even bother to help me on the till and after our shift had finished he took to Twitter to in direct tweet about me. Today I am ill and again the tweets carry on about me. I feel absolutely guilty about this situation and he has even talked about moving departments which I keep thinking why he was here first, maybe it should be me leaving, I have been toying with the idea of leaving retail for good and getting a proper career type job but the plan was to look after Christmas. Just wanted some thoughts on this situation and should I be feeling guilty. I have always been an honest and upfront person and never would I string someone along. Sorry for my rambling on just very upset and down about this whole thing.

I agree with everything except 'Better yet'. No, warn him that you'll report him, and if that fails to work - DO IT! He can damn well be the one to relocate branches/jobs since he's the only one abusing the working environment and its 'contents' for his own personal, pettily vindictive aims. Legally and morally, only he is the one at fault here ergo the one who should alone suffer the consequences of his own free-will actions. You've done nothing wrong.

He's 19 I'm 22! The thing that gets to me the most is that this is the happiest I have been in a job and now boom that's all gone on shifts I'm on with him which is most weekends and some weekdays! Plus I suffer with anxiety which pops up at the silliest of times like now so that is why I guess I am feeling guilty which I know is stupid. But yeah I think I am just going to look for something else like I've been planning and if the company want to know why ask him then he can know what an awkward situation is like.

Listen, SophieLizzie, you have a social and moral duty to report this guy (as well as to give your employer the chance to make things right for you as well as him/herself in terms of its effect on his/her business). How do you know s/he won't? How do you know this immature little BULLY hasn't behaved like this towards other female staff members before and is getting away with it simply BECAUSE none of the previous workers have dared bring it to the employer's attention? What if, unreported, he goes on to do the same to your replacement? Or (god forbid) worse? Sad to say, this may not be the first and last workplace creep you ever have to deal with in your career so, given that you're determined to leave - what have you got to lose by reporting him anyway?