I want my roommate's boyfriend to leave
Why do you want him to leave?
I don't want them to be together anymore.
It's rude to tell your roommate's partner to leave. Your roommate might get annoyed at you.
They are not going to get rid of him, that's for sure.
It's not your place to make your roommate get rid of their boyfriend, or ask him to leave. Just because you don't like him, or don't like them together, doesn't give you the right to make him leave.