Unable to remove a user from Administrators group (win 10)
There are a bunch of predefined users in Windows, which always exist and can't be deleted.
Run the following in a PowerShell session:
(Get-LocalUser -Name cBianchi).Sid.Value
look at the very end of the output after the last dash, if it is a number less than 1000, the account is a built-in account that was renamed.
For example a -500 is the local administrator account on every Windows machine.
Just disable the account if you can't delete it.