Outright theft

I cancelled my cell c contract in good time. A month after I was on prepaid my account was debited with another months contract amount. I called the call centre, the person agreed it was a mistake and I'd have to wait to be contacted by accounts ( she isn't allowed to transfer the call). After many more phone calls, they finally contacted me 3 weeks later to inform me that I've been refunded only half the debit order amount because my account wasn't debited for the 'handset fee' in the first month of my contract so they've kept the other half (about R200). Who in their right minds would believe this absolute nonsense! It's outright theft! Can anybody help? It's not right that they get away with this. It's not a huge amount but it's *****ing because they can!