Question: how to reject a boy who asks you out?
Answer: Don't make excuses. You don't need to lie. Unless it's true, don't tell him that you're in a relationship. ... 
Be straight forward, and polite. Say something like, “You seem like a nice guy, but I just don't like you that way. ... 
Keep it short. You don't need to give a long-winded rejection just to seem nice.