I'm guessing that you have a range of sub-forums and you only want people to post in a relevant sub-forum.
In that case, at the home page you need to go to:
Options > Users > Change Permissions
and remove the check mark from the "Create_topic" permission.
As all sub-forums will then inherit this permission you will have to visit each sub-forum in turn and restore that permission for that sub-forum.
Depending on the application you use you will then find that either the "New Topic" link disappears from the top level application or you will be taken to a page which has links to the sub-forums into which you may post. This list will be filtered according to the group the user is in.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.