W A R N I N G
Its not clear to me if you mean purchase or sales, but I have tested deletion of a purchase credit note and I got a problem, that you should know about.
I tested the function to delete by setting a date in the "Allow Document Deletion' before field - as described by andy4029.
I created a posted credit note and deleted it again. At first it looked fine - I was able to delete the credit note and it really does not exist in the list of posted credit notes.
BUT if I go to the vendor I first of all see that credit note is still included in the Balance and if I open the list with Vendor Ledger Entires, the credit note also exist there. The related G/L entries does also still exist. This is in my opinion all bad and I'm sure this is not what you wish to happen.
I admit I have only made one single with one scenario, but I would still say that Microsoft should remove the delete function, because the result is data inconsistency.
Also note that the documentation if you move the cursor over the label "Allow Document Deletion Before" is wrong. It says "If you enter a date, posted purchase documents with a posting date on or after this date cannot be deleted", but it is the other way around, so the "cannot" should be a "can".
Best regards
Thomas Madsen Nielsen