Over charge on contract

I have a contract with MTN. The previous month they deducted R500+ with the monthly installment I queried the issue and they claim i increased the usage but they can't seem to find it on their system when was it done. I get texts and have no access to internet when my data is depleted. They couldn't assist me to when was the change made. They opened another ticket for further investigation but the consulted said she doesn't think i will get a refund. Who else has access to my account if it's not them and it shows the last time i went to the MTN store it was in May and it had got nothing to do with this contract.