Overcharged again!!!!

I have a 2GB for R99 per month.Ive put a block on for R200 to make sure i dont get a heart attack account.The block was working and blocking but i just got an account which are already debited from my bank account of R3144!!!!.So i called MTN and they explained to me under recording that the person who put the block made a mistake and put a block on the calls for R200.They admitted that obviously there is no calls related on a data contract,but still after theyre mistake and ignorance i get a bill of R3144.The block did work previosly so MTN must have changed something on my account without notifying me cause how can the block work and then they tell me its only blocked on calls.Now i have already coughed up that amount and it seems they dont care about this expensive mistake...If this is not getting sorted im going to the onbudsman:why must i pay for their mistake.I also asked for a itimised bill which they said it will take at least a week to send,now i must worry for a week to try and source where all my money went.R3144 in todays living is alot of money,espacially if you exspect a bill contract of R99??????