Contract subscription

A call centre agent from vodacom called to offer me a contract and I thought about and eventually agreed to it, on the telephone conversation I was told I will be billed only a R100 but come debit order day R307 was deducted. I phone vodacom a couple of times but they gave me a ref number which is 1- ********** ********** . All I want is vodacom to take their phone back and unsubscribe me from the contract.