Cell taking money for a Cancelled Account

I was called last year November and asked if I wished to upgrade my data contract. I requested that it be cancelled as I no longer use it. I was informed that in order to cancel the contract I would need to call cancellations. I called cancellations in December. They confirmed Jan 2015 would be the last amount. Since then Cell C has taken off the data monthly fee for Feb, Mar, Apr (May?). In the last month Cell C upgrades department has been calling me asking me if I wish to upgrade. Every time I tell them this contract was cancelled in December, they argue with me stating it was not cancelled. I have asked Cell C to sort this out - to no avail. Please can someone higher up deal with this. I was now told today that I cannot cancel this data contract via the cancellations no. but would have to go in store. I have been a client for years, i currently have 2 cellphone contracts with Cell C. I require confirmation that the data contract is cancelled and a refund of Feb, Mar, Apr (if May) for the data contract monthly fee. Thank you