Go to your card provider (Visa/MC/Discover/Amex) and tell them to remove the charge because the service was not rendered and/or the charge was improper.
Once AT&T starts getting a lot of chargebacks, they will do something about it.
I had this sort of thing happen do me years back in NYC with Verizon. I called to cancel, was given a confirmation # and everything, and was still billed again the next month. When called again, furious, the manager I was escalated to said that they could not offer a refund because they did not have that policy. I said I don't care about policy, give me a refund, and he said there was literally no way for him to do that in the system and suggested (of course) that I accept the service for a month, since I'd already paid for it, and then if I didn't want it next month, I could call and cancel [n.b. AGAIN] then.
I hung up on him, dialed Visa, and had them charge it back. Of course THAT got Verizon's attention and a day or two later I was called by retention or some similar department to offer me a discount if I would stay on, along with a lot of apology garbage.
I told them I'd rather eat a bug.