Charged for R936 Data Used!

I'm on a Smart M contract. My bill averages around R450 a month, my account is setup to go on a R50 softlock after limit is reached. I was debited R1389.71 for December with an amount of R936.16 billed as data. The worst part is that this phone has been off completely as I've received a company cellphone hence the reason for requesting cancellation on this contract. I have opened an investigation request ref ***. Please see that this amount of R936.16 gets credited back to me as soon as possible. I heard about the strange data billings but never thought this could happen to me. I have also removed debit order from my account seeing as you have absolutely no control regarding ensuring that customers get billed correctly (even with a soft lock) Very disappointed in your system yet again!