Setup tools> Setup Wizard> Users and Security> Names and Passwords

-Highlight the user to be marked for deletion and click the delete button.

-Select yes to mark the user for deletion.

*Note: Users with historical transactions can't be deleted permanently and can only be marked for deletion.

Once a user is marked as deleted, you still have an option to see it but it can no longer be used.

If you want to unmarked it for deletion, all you have to do is remove the * character on the username.