Go to Users and search for the learner. Select the learners' name to modify his/her profile. Uncheck the checkbox titled "This user can view and self-enroll in electives" and then press one of the save buttons. After saving the users' profile, this user will not be permitted to self-enroll in any electives on your site. If the user is required to attend a live event then the user will be permitted to enroll in a session of the event.