Cell C reaping us off

I would like to raise the horrible service received from cell C. On 30 May I paid off the settlement amount for one of the phones I had on contract. On 1 June I called Cell C to confirm if the settlement amount is reflecting on their side and they confirmed receipt of the payment and the contract line moved to pre-paid. End of June I paid off the May usage and according to me I'm done with Cell C. To my surprise on the bill dated 1 July that number now on prepaid has a cancellation fee of R 984.76. My question is where is this cancellation emanating from? Once you pay off the settlement fee all other hanging charges are included in the settlement fee. Why was this extra charge never explained to me if its legit??