R 2 000 per month for data???

I have had the same 2 x 3G devices for over 5 years... suddenly 4 months ago I get charged in excess of R 1 500 for data.. Next month R 1 700 for data, I went to the call centre, was told that I need to pay and they would investigate... as of that date I asked them to block the SIM when the data limit has been exceeded, we also unplugged the router... Following month R 1 700, again went to the call centre.. the same supervisor who had previously told me she would sort the problem and call me back, was pretty rude and informed me that the query was still being investigated, she would request feedback and get back to me... Till now, no calls received - no follow up, when I went to Vodaworld a second time and asked why there was no follow up the supervisor told me she had delegated the task and had not followed up... :-(.. I'm waiting for the next crazy bill from Vodacom... it was made clear to me that it is up to me to prove I did NOT use the data... How do you do that???