Can I unassign a course that has already been taken?

Yes, you can unassign a course that has already been taken.
 Navigation: Admin > Learning > Courses
  1. Select a course from the list.
  2. Select Learners.DEV__18_.png
  3. Search for a learner to unassign the course.DEV__19_.png
  4. Click the overflow menu at the end of the row.DEV__20_.png
  5. Choose Remove Learner to unassign the course.
  6. Or, choose Reset Progress to reassign the course.

DEV__21_.png