If you wish to stop running the Payable add-on on your form, or have decided the add-on doesn't suit your needs, no problem! These are the steps to return your Google Form back to "normal".
Disable the Google Form as Payable
Open your form in edit mode and do the following:
Reopen the Payable forms sidebar by clicking the Add-Ons puzzle piece in the top toolbar, then select Payable Forms
In the Payable menu select Configure Payment Settings
In the Payable sidebar toggle the Make This Form Payable switch to No
This will remove all of the Google triggers and stop sending any automated emails on form submission or the generation of Payable order IDs.
Turning off the Payable add-on
Change your Form's Confirmation Message
You will also need to remove the Payable Forms dynamic checkout link from the Google Form Confirmation Message.
Click on Settings in the Google Form header menu
Expand the Presentation section, then Edit the Confirmation Message field and remove our checkout link message
Removing the Payable Forms checkout link
Uninstalling the Payable Add-On
If you do not plan to use Payable Forms again in the future, you can additionally uninstall the add-on from your Google account by going back to the Marketplace listing and clicking Uninstall.