Donna:

If you cleared the payment then it is no longer available to reverse. All you can do is post a deposit in BankRec as a misc cash receipt. The GL account number to use would be your clearing account. In AP post a Debit Note to the vendors account using the same GL clearing account. If you haven't run period end in AP yet you will be bale to apply he debit note to the original invoice to make it outstanding again.