IOS in app purchase, how to determine if app store user account has changed

I am creating an application that is required when purchasing an application. Everything works fine, but I have a problem.

I have a list of products to buy. When I purchase a product, I save the product ID in the default user settings, so I can provide content even if there is no internet connection.

Problem: User1 buys some products. User1 will log out of appStore (in settings). User 2 logs into the appStore. Open the application. Now user 2 will be able to see the contents of user1 unlocked.

How can I prevent this?

thank