I would suggest that whatever you do, always start at the parent / outermost layer of Nabble, because, as Camo says, characteristics are inherited.
So as Admin and with the parent page open, select Options > Users > Change permissions, and leave
blank ALL groups who are NOT allowed to Create a Topic under this application... but also consider the other actions that you may not want users to be able to perform.
Note that you will have to change this on successive pages when / if you want users to start creating topics, because permissions are inherited.
Anne