How to hide a category from users NOT in a group?

Hi,

In each product, you'll find:
- the "access level" setting which allows you to display the product only for some user groups
- the "user group after purchase" setting which allows you to add the customer to a user group after a purchase.

So with these, you can give access to new products with the purchase of a product, but you can't remove access to products with a purchase.
What you can do instead is to configure a mass action via the menu System>Mass actions. Mass actions on orders have an action you can use to remove a user group from a customer after a purchase. So that should allow you to do that.