The Credit Card maps on the Account Name and the Account Number. So to make it completely un-link from an old account:
Click Users.
Select the old User, who needs not to be linked to the Credit Card and click Edit.
Select their Credit Card set up, click Edit on the card.
Change the name on the card. For example, if the name on the Card is Joe Bloggs, change it to Joe Bloggs-OLD.
Repeat steps 3 and 4 for their Account number.
Click Save. This should undo the link between the old account and the Credit Card.
If the Credit Card is mapped to a deleted user, you can unlink it by doing the following:
Click Settings, Users.
Filter the results to include Inactive by clicking in the by Status field.
Click Import/Export.
Click Export within the Export Users section.
Open the file and search for the last 4 digits of the Credit Card in Excel. You should see the Credit Card mapped to the active user and the deleted user.
Copy the full email address of the deleted user. The deleted user will have random characters as the email address for anonymity.
Go back to Access Expense and go to Settings, Users.
Filter the results to include Inactive by clicking in the by Status field, and paste the email address into the search bar. This should show you the user that had the Credit Card mapped when it was deleted.
Click on the user, scroll down to Edit the Credit Card and change the name on the card. For example, if the name on the Card is Joe Bloggs, change it to Joe Bloggs-OLD