Digital Editions - Removing Books

How do I remove books borrowed from an online library from my Digital Editions Library?  They may expire but do not get removed or returned automatically - I just get a message saying they can no longer be opened.  If I try to delete them they just end up showing up again if I restart my computer - likely a setting somewhere, I imagine.  If I try to return the books I just get a message that they are unable to return.