Cancel the contract

I had a contract with national in fact they're still forcing the contract even after calling and telling them to cancel because I can't aford anymore. They're deducting from an empty account which I busy pilling up bank charges. Their amount keeps on getting bigger and bigger. One rude lady once called me telling me to pay the full amount when I was trying to explain to her that I want the contract canceled and would pay the amount with installments, she refused and was very rude... I regret national with their increasing amounts every month