I agree that QB should provide an easy way to delete a customer's credit card, but I found a different workaround that worked for me. I use the QB GoPayments app on my phone (which is signed into my QBO account). It is possible to delete a card from there. Open the customer profile view in the app, click the stored card, then three-dots, Delete Card. Refresh QBO on your computer's browser and the stored card is gone.
Hope this helps someone else out there!
