How can I cancel a payment made from a customer with credit card.

Hi there, @muenstermann. I appreciate your input from every angle.

If you'd want to disable the Merchant services from your QuickBooks Online (QBO) account, you can do it anytime by following these steps:

1. Sign in to your Merchant Service Center portal as an admin user.
2. Click the Account tab to display a dropdown menu.
3. Choose Account Profile.
4. Click Close Account.
5. You'll see a Reason for closing menu. Then, pick a reason and enter any comments you'd like to share.
6. Once done, select Submit.

For more information about cancelling merchant accounts, see this article: Cancel QuickBooks Payments subscription.

As for your other concern, the system or our Merchant Service Team will automatically send an email notification once customer payment is declined or disputed. If you haven't received email notifications, the email may be added to the Spam or Junk mail folder. Otherwise, the settings are not configured properly.

You can check your payments settings by following these steps:

1. Sign in to the Merchant Service Center.
2. Go to the Account dropdown.
3. Select Settings & Alerts. 
4. In the Email Alerts section, choose Yes for each notification you'd like to receive.
5. Enter the email address where you'd like to receive these notifications. 
6. Click Save Changes.

For more tips about handling disputed payments in QuickBooks, I recommend visiting this resource: Handle chargebacks and retrieval requests for QuickBooks Payments.

To help with your future tasks, here's an article that you can read: QuickBooks Payments FAQ. It contains topics that answer common questions about the Payments feature, like how to check the funding status, fix card reader issues and payments processing.

Stay in touch if you have additional questions concerning your QuickBooks account. I'll be happy to help you out. Wishing you and your business continued success.