Hi James

The control of what each role can do in a discussion forum is controlled by permissions.

The permissions can be set for each forum or at a course or site level.

Assuming you are wanting to restrict this for each forum then login as administrator and select Course> Forum Administration > Permissions

This will show all the permissions and the roles that have this and allow you to add or delete roles for each permission (there is a Start new discussions permission that you can remove for the Learner role)

Regards