How do i get rid of his ex-girlfriend?

I moved in with my boyfriend in February. His ex-girlfriend left in a huff just after Christmas, not even bothering to take her name off the apt. lease. She left him her mess to clean up. Now she is always calling and bothering us, demanding her stuff back (says she left it for him, not US), threatening to call the police. What can I do to help him? Does he have any leverage if she just left her **** and left?? Thanks.