Can I tell my roommate I don't want her boyfriend staying over anymore?
Why are you going to tell your roommate her boyfriend can't stay over?
I don't want to share a living space with a person I don't like
Roommates are expected to respect one another's privacy. If you tell them who not to have over you could upset them, especially if its for a petty reason. Try finding somewhere else to be while your roommate has someone you don't like over.
It's not like I'm kicking her boyfriend out. They should be mature enough to be told who not to have over.
If he is starting to live there and doesn't pay rent It seems unfair to you. Have you considered looking for a new place to stay? Or finding a new roommate?