Tikona — Discontinue service still bill generated

I have stopped using Tikona services this January. Still I received bills for payment eventhough data usage shows zero. My plan was prepaid so when I stopped service there should not be any money charged. But recently I got legal notice to pay certain amount to them. Even after disconnet mail they do not respond. While joining connection we bought our own devices for home. So I do not get for what they are charging money. I atleast need some explanation which they ar enot giving,