Currently the only way to turn off Autopay is through an invoice email, typically done by the customer/client directly.
To do this on their behalf, you simply need to send an invoice to yourself.
Log in and navigate to the invoice list. Find an invoice from the customer who needs Autopay turned off (use Search criteria to filter by that customer if needed).
Select any invoice that is not canceled (all other invoice statuses are able to be emailed).
Select the Email invoice option. Change the email address to one that you have access to and select Submit.
Navigate to the inbox to which you sent the invoice. Within the email, there will be two options. Select Turn off Autopay.
On the following page, confirm the choice to turn off Autopay.