If you'd like to remove the assignment for a user, you can do so, as long as the user hasn't started with the course.
Navigate to the Assignments page.
Click on the Manage Assignments button for the course that you'd like remove the assignment for.
If the user hasn't started with the course yet, you will see a Bin icon next to their name. Click this Bin icon to remove the assignment.
Removing the assignment doesn't remove the seat. That means that you can reuse the seat for another learner.
Note: the user whose assignment is removed will not be notified about the removal of his assignment. Also removing the assignment, does not remove their user account.