Airtel — wrong deduction of balance for about last two months

My name is Shahid Mansuri. I have sony ericsson K300i handset and sim card of airtel. About two months ago I had activated my gprs service and after few days i had deactivated the same. But yet now there is deduction of charges. About 10 times i have called to customer care. Every time they ensure me that money will be refund to you for the wrong deduction. Sometimes i got refund but the problem remain as it is. i dont know what to do? According to customer care, there is some technical problem. but why i'll have pay for this?
Many a times i have requested them but nothing changed.
Please help me
I'll be grateful to you if you can solve my problem.
My name - Shahid Mansuri
Address- Sutar mohalla
Nandurbar (maharashtra)
Mobile no.- [protected]