I have the same problem. I want to delete myself as a customer in ZC, but it does not work. I have lost the login password for this old account & need to create a new account so I can check to see how my PayPal is working. So I go to Customers tab, click on Customers & highlight my name, then click “Delete” and am asked “Delete Customer - Are you sure you want to delete this customer?”, I answer yes, but the account never IS deleted, it continues to appear in all areas. I see your comment “Sounds like you have installed the recent security patch except for the updated /admin/includes/functions/html_output.php file.”, but I do see this file is there. Can you clue me in further?