I want to only allow the ability to add WBS elements below this 9 phase WBS? i.e. Level 3 and below?

You could grant WBS security to the ones you want people to have access to with assigning them in the responsible manager. This means that these users are assigned to a group that is then assigned to the WBS that you want them to work on. The administration of doing so can be tedious and create more work as each responsible manager in the WBS then has to be maintained.