Adding and removing accounts can only be done by the account owners and can only be done by entering credentials for the accounts involved. As such Bungie is unable to link or unlink accounts for players as they don't have access to the log in credentials. If an account has been added to your profile that you didn't do, that means someone has access to your platform account log in credentials as that is the only way they could have added their account and enabled cross save. Unfortunately if you cannot obtain the log in credentials for the unknown account then your profile is stuck with its current set up and you will not be able to disable cross save. It also means someone else will permanently have access to your D2 characters.