I wouldn't have complained a the main office. I would have called the police and told them that the mechanic performed unwanted service on the car and was falsely detaining you. They're supposed to give you a form to sign authorizing the service. if you didn't authorize it, it's not supposed to be done.

Had to call the police once from a service center in........name redacted to protect the guilty........ It took me 9-1- and I never got to the second 1 before the manager was speaking with me.

If you're milquetoast, you're going to get burned.