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!