Since he paid you rent, legally you are the landlord and he is the tenant. That means you have a right to have a key to their door, and must give notice before entering, and must evict him formally--you can't just boot him out, if you do and he calls the police you could be in serious legal trouble.
I'm 22 and have had friends go through this kind of "phase" before--the best thing you can do is act as though you support the relationship (if you ban her from seeing him he will suddenly seem like the most interesting, "forbidden" bad boy around, trust me!) and wait for the relationship to die on its own. And do whatever you have to to make sure she is on birth control! Make her get the shot or something else very effective and long-term. As long as she doesn't get pregnant this will go away on its own.
