Excessive data usage when modem was unplugged

I have a R99 data contract with Vodacom. Last month R854.12 was deducted and this month R738.12. I queried this with Vodacom but they insist that the billing is correct. I requested that a block be put on this account on 02 July 2015. The modem was not even plugged in during the times that they state on the query information. I requested that I must be refunded and after a sarcastic laugh I was told that it would not happen seeing that the data was used. Vodacom cannot explain to me how data can be used if the device is not even plugged in. I received an e-mail from Vodacom stating that the billing was correct. By looking at the amount of similar complaints on Hellopeter, Vodacom should start shaping up and fixing their errors and attitude towards their customers. The ref nr supp**** to me by Vodacom is S3-8d02i-iidte. I am not prepared to let this go. Further action WILL be taken. Just fix your mess and pay more attention to customer service than enriching yourself in this way...