MTN keeps on debiting me even though i returned their device and cancelled the contract.

I took a cell phone contract online with MTN in May. 2 Days later i returned device to the nearest MTN store because it was not accepting dual sim. MTN promised to cancel the contract. It has been 6 months now. When i stop the debit order the change the reference number. When i call the promise to cancel the contract and nothing happens. There are ***** deals going on there. I want all my money refunded.