The code you developed for your client was most likely never yours to begin with.
Bingo! Go back and read your contract. No contract or there's no language in it about who owns what, then it's theirs (If you're in the US) and they can do any damn thing they want with it. It would be straight up madness if a client hired you to do work and then they couldn't change the code without your permission. (And yeah, changing comments does fall under that.) Even if you put your own copyright notice on there, unless the contract specifically says otherwise, you've handed copyright over to them in doing work for hire. (IANAL, but I've been a contractor for 24 years.)
Try contacting the people who hired you and tell them to change the copyright notices to something more correct, but I doubt if you'll get any joy there. They have no reason to.
Lesson learned, move on, and for the future make a contract that reflects your wishes.