Be very wary of Vodacom *****sters

I am a Vodacom Business client and have been for many years. A while ago I realized that my monthly debit order is much higher than is should be. I live far from their nearest branch and phoned the call center to query my account. After holding for very long I was informed that I subscribed to a third party service in terms whereof they charge R12.00 per day. They could not confirm what the subscription was or how I subscribed. They further confirmed that is a third party and has nothing to do with them. Eventually they agreed to cancel the subscription and refund my money. It is more than three months later and I have called several times and drove far to visit a branch but still no refund and they just keep on denying liability. My question is....if you grant a debit order to someone for a specific service, how can they increase this debit order without your consent on behalf of a *****ulent third party and just deny liability?