First, click on Users & Goups in the navigation bar. Next, click on the Edit button beside the user you want to delete.
Lastly, click on the Delete User button in the window that pops up. Accept the confirmation, then the user will be deleted!
NOTE: USER DELETION IS PERMANENT. WHEN A USER IS DELETED, ALL OF THEIR COURSE PROGRESS, HISTORY, AND USER INFORMATION WILL BE LOST WITH NO WAY OF RECOVERY.