Unable to stop contract

I have a contract at cell c which expired in March already. I have phoned multiple times to stop the contract since I no longer use the sim card or cell phone. The third time I phoned them they said the contract has already been stopped but they will stop it again I have phoned them another 2 times after that and every time they say the contract has already been stopped but it will be stopped \again". I have even stopped the debit order at my bank but money is still deducted from my bank account. I keep paying for a service which I don't use and I am tired of struggling to cancel the contract. """