Tata Docomo - Raising undue bills for broadband service after disconnection

I had written to Tata Docomo to close the account as they were not meeting the service level they promised. The connection was frequently interrupted and the speed was too low. On receiving the mail their office requested to wait for a few days till their service engineer visit the home. During his visit, he accepted that they will not be able to meet the expected service level and accordingly we advise him to close the account however, this was a verbal communication. This delay from Tata Docomo led to the shifting of timelines to the next bill cycle and we paid for that. Subsequently, we took the connection from another service provider and paid to them also for the same period.
Tata Docomo did not close the account and raised the bill for next bill cycles and there is constant follow up from their office which has reached to the level of harassment as they are regularly calling in office time.