Most Helpful Guy
If you dont know the guy then the boyfriend line is the best, no feelings hurt - if you know the guy you just need to be honest and say you aren't feeling it. It might hurt but its bettrr to know straight out there's no chance than to be told "yeah im not ready at the moment" or similar then feel there's still hope
Most Helpful Girl
You could always say you are enjoying single life right now, and have education/work commitments that are currently prioritising your life... although, whenever I've said "thanks, but no thanks" I swear it gives them some sorta incentive to work harder. So sometimes a white lie about an imaginary boyfriend is the best answer...