impress upon him the gravity of the situation....with a board upside his cranium.
but seriously, let him know this is a big deal and if he is kidding he needs to quit.
I'd be disturbed too dude, nce my roomie picked up this panhandling junkie. (didn't know he was until he came over, never hit on guys in the park!) total pain in the ass and he kept calling for a week after, I was so paranoid he would come over some night and take our stuff.
Next guy to come over goes through a thorough background check...and frisking...heh