I'm not going to let a friend's friend stay with me
Is there a reason that he can't stay with you?
Because I hate him. I think he is a loser
We are all allowed to have opinions of people, and you are right to stand your ground and not let just anyone stay over. It is your home and your property, so that friend of a friend has to know to not overstep their boundaries. You don't know if they're a killer.
I hate him, but I don't want to hurt his feelings.
If you do not feel comfortable with him enough for him to stay at your house, then do not do it. You are not required to open up your house to people that you barely know.