Honesty is always the best policy unless there's a chance that you might get laid. Then getting laid is the best policy because you get sauce on your willy.
So if it's someone you can do sex to - and by can I mean do they have a hole that opens - you should stop being a virulent pussy and let that fucker stay with you. Otherwise, woman up and be honest about that shit.
I've let people stay with me before when I don't want them too. It sucks. And they didn't blow me at all.
Short Answer: I know being honest isn't funny, but it makes the world a much clearer place.