AT & T MOBILITY or AT & T WIRELESS MAKES YOU PAY FOR SERVICE THAT HAS BEEN CANCELLED Nationwide

I cancelled my wireless cellphone service on September 16, 2015 by phone, to their agent,  because I changed carriers.  AT & T shut off the wireless service on that date and I had no cellular service at that point with AT & T.   I was told I was paid up to date and, in fact, would receive a credit for the $5.00 late charge I sent.  Next month, I receive a bill from AT & T stating I owe them $98.92 after they applied the $5.00 credit from the previous month.  This amount covered service from September 16, 2015 to October 15, 2015.  They bill you for service before you use it, which I was already aware of from the previous month's billing.  I called them again, and the Agent said no, that was not correct, I owed NOTHING because the service had been turned off on September 16th.  He asurred me it was all taken care of and I would receive a bill showing I owed nothing.  I then received a bill for $103.92, which was a late bill which included a $5.00 late fee!!!  I called them November 3rd and was advised I DID indeed owe the bill and it was late.  I asked them how I could owe for service that was shut off and I could not have used if I wanted to?  The Supervisor's response is "because it's the billing cycle" and they do not pro-rate the last bill.  It's in their "contract" he said.  OK, so AT & T can make you pay for service they have turned off at the start of the new billing cycle and you cannot use????  This is legal????  I paid the bill because they would have messed up my credit rating but this is one corrupt way of doing business.  I could understand if I had been seveal days into the billing cycle and I had been able to use the service.  But this is just wrong.  Buyer Beware!!!!  Don't use AT & T unless you don't mind paying for nothing.