Data Overcharge

My account was debited with 4 times my monthly installment and the explanation I got was that I used a lot of data, during the period this alleged data usage took place my phone was soft locked as I have a limit on my number. Vodacom has confirmed that my line was soft locked and I could not not use it during this period. Now Vodacom is telling me they need until the 17th to investigate which I don't understand as we are all in agreement that I can't use data on a soft locked phone. I've even been to my local branch to ask them to check the data usage on my device. The call center says there wont be a refund instead my account will be credited once the investigation is complete which I find ridiculous that they can just wrongfully take my money and expect me to get it back on their terms. I desperately want to cancel my contract now but I know I'll be held ransom with an insane cancellation fee even though they are at fault.