I have now invested over an hour trying to cancel my subscription. The customer service agent (Overseas) told me the only way to do so was to cancel my credit card.

Hey johnwsprecher. Cancelling QuickBooks Online should be a simple process. Just follow the steps below to do so and here's a video on managing your billing. 

1. Select the Gear Icon in the top right.
2. Select Account and Settings in the leftmost column.
3. On the left of the screen, select Billing and Subscriptions.
4. Then, select Cancel in the correct subscription box on the screen.
5. You'll be presented with a quick survey and a confirmation to cancel.

After that, your subscription will be cancelled. Let me know if there's anything else you need.