By default, a News Forum in a Moodle course will allow posts to only be added to it by teachers on that course.
All other forum types allow all participants to post messages. This can be overridden to prevent students from posting to a specific forum.
- Within the forum in question, click on the Permissions link in the Settings block.
- Find the relevant activity (e.g. Start new discussions, Reply to posts) and click on the red cross to the right of 'Student' to remove their permission.
- Check the permission removal information that is displayed and click on Continue to confirm the change.