Thank you for posting! It is not currently possible to prevent test cases from being added to specific sections or subsections. Any user that is assigned a role with add/edit permissions for test cases would be able to add new test cases to any section in the test suite. We generally recommend using the user roles and permissions when possible to restrict and limit access to specific functions in TestRail to a small number of users to prevent mistakes. Users and their assigned roles can be managed under Administration > Users & Roles. You can find more information about managing permissions and roles on our website here:
We are happy to review adding support for additional permissions options in a future update. In the meantime, I would recommend using the description field of a section to add a note to remind users not to create test cases in that particular section.
I hope this helps!