Airtel — unnecessary threatening and ultimate legal threat by airtel people

I am an old customer of airtel and things were good till the account closer. An extremely awful experience like many other airtel customers when account cancellation is concerned.
We informed airtel people repeated times that we are not using this connection, hence kindly stop the connection and billing. But the billing cycle is continued and repeated harassment calls we have received from their team for bill payment.
The ultimate call we received from them when a person claiming himself a lawyer said that they are going to file 420 against us within 1 hour window and we need to appear at delhi high court next day just because we have not payed the bill for 2 months (Though we informed before to stop the connection and billing). I really can't digest this cheap stunt by such reputed company. After this call i am also thinking if there is a forum who volunteer to file real cases (Not only 420 but also the penalty charge for harassment) against who does such cheap policy to increase their wealth by bleeding their customer. Bill till date is already paid as people like us don't like legal obligation so they use this soft nerve. Being a customer we can't suffer this harassment anymore, so i mailed them mentioning it as an official request to stop the the service immediately and we will not be liable for any bill henceforth after this payment as already informed that we are not using it anymore from past few months. 121 is dumb way, there should be some online facility to take cancellation request to bring the transparency in this process.