LazyPay — Unauthorized bill

I have been charged Rs.425, (which is Rs. 566 now because of late charges) by lazy pay on behalf of hathway which I did not approve btw. Customer care from lazypay have been calling me daily thrice a day asking to pay the bill and I have been constantly telling them that I did not approve the transaction. It might be a small amount but I'm not going to pay it if I did not approve, that should be obvious enough. But no one seems to understand this and keeps on pestering with calls.