Ignore the guy that you don't like. Ignore his phone calls to you and don't call him back. Delete any emails he sends you without responding to them first. If he sends you text messages, simply delete those as well. If you ignore the guy for long enough, he will hopefully get the idea that you don't like him and don't want to be romantically involved with him.
Act mean towards the guy that you want to stop liking you. You can make fun of him to his face or in front of his friends. You can laugh at him also if you really want to hurt his feelings. You might also opt to say something nasty about him if you want to alienate the guy. Only act this way if you no longer wish to even remain friends with this guy. Be careful, as well, because you don't want to get a bad reputation for being a mean and heartless woman.
Find a new boyfriend. Be sure to go out on very public dates so the guy that you don't like will see you. Make sure you have a lot of fun on your dates as well. If the guy you don't like sees you having fun with someone else, he may stop wasting his time trying to pursue you romantically.
Be honest with the guy. Tell him that you simply don't want to have a relationship with him. You can suggest that you remain friends if you wish to go that route, or you can ask him not to contact you any longer. Try to be diplomatic, kind and tactful when you talk to this guy. Try to talk to him in person, if possible, or over the phone. If you feel uncomfortable with talking in person or over the phone, consider sending the guy an email.