Airtel — Postpaid to Prepaid moved to USA

Hi Sir,

I moved to USA few months and requested Airtel to change my post paid mobile service to prepaid. Customer care is not at all responding and at last they said that customer need to present in person to change from post paid to prepaid. I am not using Airtel from few months and they are sending mail to pay the bill every month. Can you please help to change my post paid service to pre paid.

Thank and Regards,
Vinod