Airtel not disconnecting postpaid number and sending bills

I relocated from Hyderabad to Delhi in April and stopped using my Airtel Postpaid mobile number – 8897508467. I contacted the helpline and they assured me that they will deactivate my simcard. Despite this, i kept receiving bills every month. I wasn’t contacted to tell me that it had not been deactivated and they kept billing it to me. In July, I got a call and was told that I had outstanding balances. After bearing the extremely abusive and uncouth behaviour of the uneducated Airtel representatives, I was told to pay my outstanding amount of Rs. 2,300 at the local Airtel Store, which I duly went and paid off that day. I then provided the transaction ID of that payment to the female Airtel representative on the phone and she assured that my phone would be deactivated that very moment and that I would have nothing to do with it from then on.

But the bills are still getting sent. Every month 800 rupees are being charged to me despite the number being out of service(when you try and call it) as rental charges + BlackBerry services. The number was NOT DEACTIVATED DESPITE THE VERBAL CONFIRMATION. This has now accumulated to more than 4000rs. One of the bills even shows a credit of the 2300 I paid. Today, on 18th Dec, I get a call again asking me to pay my outstanding amount. i don’t understand why this is happening. Why was my number not disconnected in April and then again in July? And why was I, at no point, informed of the non-cancellation. And why I am I being contacted in December, after July? The fact that they wait for so many months clearly shows that Airtel has resorted to extortion. Airtel is surely the worst telecom service in this country.
To Airtel_Presence, kindly resolve this issue as soon as possible. I cannot stand this humiliation anymore.