Deleting Mailboxes with no eDir account

I have just done a major cull of students who have left the college, and I used LDAP (Apache Directory Studio), cos it is so much more flexible than C1. Now I have a bunch of GW2012SP1 mailboxes that are orphaned, no corresponding eDir account.
Is there a way to flag, or show these mailboxes so i can delete them? Or is there a way to run some sort of maintenance on the PO to achieve this?
Thanks.