overcharged my account

I have a R99 2 gig data bundle with cel c , may I got billed R 1500 , cel c only responded that I went over my 2 gig bundle ( which is impossible ) after requesting proof they couldnt provide any , I took it as school fees and decided that I will not use cel c again and put the simcard in a drawer , I never even touched that simcard in july yet the bill me R 997 for june , after phoning cutomer care the once again yild me ive used more that my 2 gig , that is utter nonsense as I did not and they said they cant proof where ive logged on on that simcard , my question is if they cant proof that I used that data how can they bill me ? <br> now I know how they can offer R 10 000 to do contract buy outs , its because of them robbing their excisting customers !!