Moving Nil Value Invoices To Paid

When I create a NIL value invoice the system usually moves them straight into PAID - however, I have a few invoices that are NIL value which are still showing on my UNPAID list. Although they don't show on the Ageing report, I would still like them to move into PAID just to make it accurate. Any advise on how I can move them? At the moment I have them showing as PAID Pending on UNPAID.