Enter credit memo against a fully received/fully matched PO

My users have the need to enter a credit memo (negative receipt) against a fully received/fully matched PO.  The system will currently not allow for receipts to be entered against a fully received/fully matched PO.  The workaround they have developed is to enter a direct pay voucher for the credit amount without referencing the original PO.  However, this makes it difficult for some areas to reconcile their high volume of encumbrances.  Is there some process within the system that they should be using to accomplish applying a credit memo to a fully received/fully matched PO?  Ideally, the status of the