Can an ex-tenant be banned from visiting a building because he owes money to the landlord?

Your friend can get in trouble for not paying what he owes, he should consider himself lucky that he hasn't been taken to court yet. With that said No he cannot be banned. However it is private property and refusing to leave would get your friend in trouble, and you may be possibly evicted if you continue to give your landlord a hard time. The best way around this is to hang out with your friend elsewhere, it's better to just avoid any conflict than to stir up trouble.