Still being charged after cancelling my contract

Towards the end of February I went into the Greenstone Mall 8ta branch and cancelled my data contract. I gave him all the relevant documentation as well as completed the required form. He told me I would still be billed for March as it is one months notice, which is fine. However, come April I am charged my subscription AGAIN!!! I emailed the customer services department and I got no response. I think this is absolutely disgusting service from them and they are complete *******!