Easy! Begin by clicking your initials or photo avatar in the top right corner and selecting the Users option from the dropdown menu.
Click on the row of the user that you want to delete. Their profile will expand below. On the bottom left under the user profile, click on the text that says Remove User.
Anow will ask for confirmation to delete this user. Click "Delete User" if you're sure.
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.