Vodafone India — postpaid bill

Address:

Chennai, Tamil Nadu, 603004

I had been using vodafone postpaid connection for more than a year. Then i wanted to drop the connection and i asked them to discontinue the service. They said they would initiate the process.
What happened next?
They send a bill for the next month. I didn't use the service but they sent a bill. They sent another bill on the next month again. Then they started calling and saying that i had to pay the bill amount and threatening me. I simply denied to pay but they said once you pay the bill for the last time, we will discontinue it otherwise we will send you legal notice to your local address.

I had to pay unfortunately. Then on the very next month they sent bill, i didn't pay. Again they said the same story, if i don't pay they will send me legal notice.

I am really frustrated and scared now, i don't know what to do. I have never seen such poor service provider. Please help me out and take necessary actions.