To remove a payment assignment go to your Club Control Panel click on Shop and Payments and then click into products.
Click on the product in question.
You will then be taken to the Assignments page for this product, this will show you everyone who is assigned to this product.
To remove an assignment from the list, click on the 3 dots at the end of that assignment and click Remove assignment.
You will then see that this assignment has been removed from the list.