Easy! Navigate to your Account Settings by clicking on your initials, the clicking the Users tab.
Click on the row of the user whose role you’d like to change. Their profile will expand below. On the bottom right, click on the red text that says Remove User.
Anow will ask for confirmation to delete this user. Click yes if you're sure.
This user will still be displayed in your users table, however it will have a
flag. All files they were associated with will remain with their name in tact for your records. However, they will no longer be able to login, and you can no longer assign them to any files.