Vodacom adding stuff to go off my account!

So after i have received a bill from Vodacom for the past few months i have seen that my debit order has gone up with a certain amount each month. I then contacted vodacom yesterday and the one lady helped me and she said that there was a R99 month to month bundle added on my contract. I have not chosen to get any data month to month as i always buy my airtime via Prepaid24 and then i convert my own airtime to data. So today i sent the enquiry for the money that was deducted for that to be refunded to me as i did NOT activate any data to be added onto my phone per month. Agent tells me that he can see that there is a R100 purchase made on the 28th of May and just after that there was a data bundle also activated. So i say to him, WHY would i buy airtime to convert and then just activate another one on their stuff. He says to me that there was a client that did the same as be and do i know what a USSD string is...... i have been converting my own data for years!!! I know exactly what a USSD string is! I think that you stuffed up, when i bought my airtime and converted it R99 for 500mb - you converted that one and added another 500mb to go off my account! want my refund!