How do I remove JotForm Ad after form submission?

I believe you are referring to the JotForm footer in the Thank You page.

The only way to remove the footer is to upgrade your subscription to a paid plan. Once upgraded, you can then uncheck this on the Account Settings page in order to remove the footer.

Here's a guide on How-to-Upgrade-Your-Account.

Let us know if you have any questions. Thank you.