Horrible service from Cell c and I want my refund

I have been having a horrible service with cell c. An agent calls bme and sells me a similar card. He says it's R129 for 20gig, 10gig a night and 10gig a day... Never had he ever told me it was an open line after I have asked countless times if I won't be deducted any more than that...on 25 Sept finally an agent calls and tells me that they can see on their side they have over debited me with R900 and waited for finance to approve...on top of that they debited R305 on my account on 25th and now again today 30 Sept they debit R791.10... like how on earth do u debit twice after you notice a fault on your side? This is day light *******.... I want to cancel my contract with you, you obviously don't know how to treat customers with respect...and stop this thing of selling stuff to me when I want to cancel instead of doing just that.