If you are a admin, you can set the permissions for the parent community to not allow posting of content by everyone. To do this, go to admin console --> Permissions --> Space Permissions and Edit the default space permission.
With that option gone, people will have to choose a community or create a private discussion. If you want to disallow private discussions, you can add the following system properties (depending on which content you want to disallow). Please note that it affects all users.