I'm trying to find a humorous term for someone who still has an overdue library book after a number of years. I first came up with "delinquent lendee", but I'm sure there's something more apropos or alliterative.
Assuming they have no intention of actually returning the books, you might call them biblioklepts.
Perhaps scofflaw could be appropriated for this use. It's probably what I would say in the same situation, off the top of my head.
(Just checked to make sure you're an American over 30)
How about Costanza or even "Can't Stands Ya".