There are multiple reasons why you might not want your clients to access a certain app. Maybe you want to be the only one to track their progress, you don't want them to create their own training plans or your company does not want to work with nutrition.
To make sure your clients can't use those apps, you can disable them. You do this in the system settings under portal content. Please note that if you disable an app in the system settings, it is no longer available for any of your clients.