My mother is freeloading on her ex-husband's rent and space?

I would just be straight up with your mom. Invite her over, sit down and have a one on one conversation with her. Just tell her that shes being selfesh in so many words. Be rational, be reasonable but tell her everything you feel even if its a little harsh. Just be dead honest with her and hope for best and if she still doesnt listen atleast you can say you tried.