I can't imagine all the emails my friends would be getting after that conversation.
That is extremely creepy he messaged your future roomie to see if he can sneak in and live with you guys. Man just straight up be like I dont like you messaging my friends about me, there is nothing and will never again be anything between us. Nothing against you, you're a fine guy but I know you are not the one for me.
I never understood these people though, if I got treated like shit I'd walk off and stop trying to be friends with them, why would you bother trying trying to make friends who are assholes, but they just keep following people around like a bad smell.
Ignoring the problem won't make it go away, and telling him strait up didn't work. sounds like harassment to me and if you don't mind my opinion I think you should go to the police and get a restraining order against his ass.