Still paying contract after expiry date

A few weeks ago I got a call to upgrade and I informed the lady that I will no longer need the contract but they're still debiting my account and if I don't pay they suspend my phone. I don't know why do we need to explain ourselves when we signed a two year contract. After the expiry date why do they still feel the need to debit my account.... I will never again use this supplier and I will definitely not recommend them to anyone.