Unauthorised debit of account

Vodacom entered into a contract with somebody other than me and debited my account during October 2014. I immediately queried this, they phoned me and stated that the contract would be terminated, the issue would be investigated by their forensic department, they would provide feedback to me and refund me. Since then several follow ups from me were ignored and I have not been refunded. <br> <br> Today they debited my account again for a substantial amount of money, apparently for the same contract that they supposedly cancelled. This is unacceptable!!