Normally, Members can submit content to their communities, and Managers can approve (or exclude) that content for consumption by the community. Submitting content is a permission, and can be removed from Members as needed.
To turn off the "Can Submit Posts" permission on an individual member's profile:
- In the manager app, go to Members > Members
- Search for the Members name and click it
- Click Settings
- Uncheck the box next to "Can Submit Posts"
- The next time this member logs in, they will not see the Submit Posts button.
Also, the default setting for newly invited members can be changed under Admin > Registration > Registration Settings (see attached screenshot).