Unfair charge

i took out a contract with cell c in December, apart from the network having issues i paid regardless, i then spoke with a rude person from the call named Derick who was rude and told me i cant cancel and i must pay their money. I then went to the store where i was advised that i cancel the contract on the spot, i returned the phone to the store on 26/04/2015, i advised the agent that i do not want the sim card and he advised that i can keep the sim card and he will change it to prepaid, i spoke with the agent via email where i specifically told him that i do not want to get charged for this service, he assured me on the email that i will not be charged. and funny enough i received a sms 16 days later 12/05/2015 telling me that my contract has been cancelled. i have since been charged again for that service that i have not had for a full month. please advise as cell is getting to my last nerve. i would never recommend cell c to my worst enemy and i now know how cell c buys other peoples contracts because they debit unsuspecting customers money out their bank accounts. on sunday 07/06/2015 i was promised a call to sort issue out no one gave me a call.