Airtel — Airtel not disconnecting the connection and putting extra charges

I took a connection with a one year rental (rent rs 699 of one year) plan with airtel.
Everytime I had payed all the bills on time. When one year was about to complete, I asked Airtel customer care to dsconnect the service as I wanted to take other subscriber with lower call rates. But instead of disconnecting, Airtel changed my plan to Rs 300 rent per month with same high call rates.
Because of this I again called customer care and told that I will not pay the bill if the rental was not deducted and disconnect the connection but they did not disconnect it for next three months.

Now, I am getting abuse calls from a person called Deepak with number [protected]. Who uses false language and tells me that if I will not pay the bill then "wo mere poore khaandaan ko court main khasitega" etc.
I am still ready to pay the bill for call chrges but not the rental and late fee.