My advice to you would be to tell this guy that you're not interested, you have a boyfriend etc (even tho you don't) and see if that gets him to back off. If it doesn't, either take the matter further to a manager or find a new job. I know that option is a pest but it will mean that you can work somewhere free of being pestered. No one deserves to put up with that kind of behaviour at work and you certainly don't need it.
I agree with several posters on here regarding how women are objectified. I'm the only young female in my workplace under the age of 30 and work with men. I've had a few borderline sexist comments made at me and I've had to either grow a set and shake it off or rather, sit and silently fume. I got told the other week that what I'd put on my personal development wasn't going to happen as both managers wanted to me go forward into a more managerial role as they thought all the guys would listen to me more, because I'm a girl. I have no interest in a managerial role. I'm actually looking for a new job as I find the male testorone levels can be too much and I'm simply viewed as the "pretty one that sits in the corner".