Restricting enrollment by course disqualifier prevents users from enrolling in a course based upon their previous completion of another course in the system.
...
- Navigate to the course.
- Click "EDIT."
- Click "Pre-requisites."
- Navigate to the 'Course Access' block
- Under 'Type' select 'Enrollment Blocked'
- Select a course to block in 'Course' field
- Click "Save."