Asking a payment after full and final settlement of closed broadband connection

Location/place: Ghaziabad, UP

Name of company/service: Reliance Communication

I was raised a request to close my broadband connection on Mar 19, 2013, because most of the time their network get down and when I call CC to register complain they said its a issue from there main system and they can’t register this request and connection will be get normal in 24hrs, I was get frustrated by their such excuses.
By Mar 24, 2013 my connection was get closed and on Mar 30, 2013 one of the reliance executive visit my home for full and final settlement of my bills.
I had paid Rs280 as full and final and got the receipt for that. In next one week I got the NOC letter by post from Reliance as well.
But in May 2013 Reliance executives start calling me and saying that I was use their connection till April and so I have to pay the bill for that, when I asked the bill they did not call me for next 20 days and neither provide any bill as well.
Now On Jun 12, 2013 Reliance sends a bill on my mail id and asking an amount of Rs610 as a full and final settlement.

Anybody can tell me what can be done in these scenarios, when I have a full and final payment receipt from reliance communication of Mar 30, 2013 and a NOC letter from their office as well.

Many Thanks