Vodacom is terrible

My contract came to an end and we opted to cancel it completely however Vodacom are still billing me. Call centre say it is cancelled but the debits are still going of. No manager returns the calls and we still sit with paying for a contract that came to an end and we opted to cancel. Becareful of taking contracts with Vodacom as they dont want to end the contracts although the contracts are over. What can be done about this. They should learn to call clients back.