Just tell them in your reply that you have already raised the request on ** date and its their fault for not disconnecting.
Ask them(In Bold) for the reason why they took this much time for processing your request and unless they answer you are not going to respond further.
.Also SAY in bold latter that You will post your story in various website related to Consumer court and if still they don't stop then you have no option but going to consumer court.
PS. I had similar problem with Vodafone postpaid, and the above 2 lines did the trick. Here after 4 years again I'm with Vodafone Postpaid connection.
With Airtel Internet connection - I simply forgot about them, sent them an email and then a reminder(They had my prepaid number which I stopped after shifting to new city and never bothered to check emails
), didn't even retrurn their phone/modem as never got any calls for any person to pick them up, this was 2 years ago.
With My last ISP local cable 6 months back - Literally told them to f**k off and not to call again. I'm not in city, so do whatever you want. Didn't even pay last bill amt and told them to adjust it with security deposit.