Overcharged

I have 3 contracts with cell c and one of them is coming to an end, I have been receiving 2gig data on the one that's about to expire and later on cell c added 3 gigs more of which I never used I barely use the 2 gig that I initially took when I received the package, but my issue now cell c is overcharging for this and this was not even communicated to me, I after receiving my statement I called cell c to query this but that did'nt help I even tried to stop the debit order from going through so that I could pay what I actually owe and what was agreed upon between and cell c but they still deducted my money. Cell c is a very bad service provider and I am not happy at all about this even when I talked to one of their agents she said she does'nt understand why they overcharging because the additional data was supposed to expire together with my contract at the end of this month May 31 2015, I want my money back I mean they always say all calls are recorded so I want them to pull out the record where I agreed to additional data that now I must pay for , and the fact that I have never used the stupid data ,in the first place does'nt even count.