In order to restrict a category, select it in the subscription plan-> Restrictions tab:

 

category_restrictions

This subscription plan restricts 4 Joomla Content categories: FAQs/Current Users, FAQs/New to Joomla!, FAQs/General and FAQs/Languages.

To make the restrictions work, the Content Controller plugin must be installed and enabled.

 

category_restrictions2

This subscription plan restricts  4 Joomla Content categories and 4 Mighty Resources ones.