Charging for unused data

I have a data contract with Cell C for R99/month. I get billed monthly. In the past 2 months I got billed in excess of R800 for a service I do not use as the tablet were packed away and not switched on during this time. I surely know now that the data reflection on your account is surely not your actual usage. I have snipped up the sim card and send images to Cell C to assure not to have the same issue next month. I would rather pay R99/mont for the remaining term for nothing then get billed for data I do not use.