Why don't guys give up when you express disinterest?

Heh, men are very motivated creatures and it can be hard to get them to back down. Tell him up front, in the most direct way possible, border-line being a total b* tch you are not attracted to him and you do not want to be with him. If that doesn't yield better results tell him you're into women and he could either understand or want you more. It's a gamble really.