Airtel — Airtel - International Roaming Deposit

I have been harassed by Airtel for getting my Rs. 6000.00 refunded which I had paid to activate my International roaming. I had paid this amount in August end and when I returned back I requested to cancel this service and asked Airtel to refund this amount back to me. However, after waiting for 45 days when I called back the Airtel call center, they told me that they have not received any deposit for activating my international roaming 1) I have the receipt of the transaction that I made 2) My international roaming was active while I was abroad. - is it possible for them to activate this service without the deposit? The call center people took the details of my account and the receipt that I had got. After furnishing the details I was told that the amount will be refunded back to my account. The next call that I got was surprising. They told me that the amount is now credited into my account. This is just 4 days ago. And the service is activated. I then went to the Airtel office where I had deposited the amount and they told me that the amount is now reflecting in my account. And will make arrangements for depositing the amount in my account within 7 days. I submitted the letter and the cheque details the next day. After waiting for 3 days when I called them again they told me a different story. Now it will take another 45 days to get my money refunded. I was furious and called up the Yeshwanthpur Airtel office where I had deposited the money. They are telling me that they are busy and will call me back..... The story still continues. I am not bothered about my 6000.00 now but want to teach them a lesson so that this does not repeat again. Please suggest Airtel Rs. 6000.00 that I had paid in Yeshwanthpur Airtel office has