Dstv is refusing with my money

Hi Guys I have a problem with dstv they called last month offering me an explorer at the price of R499 monthly,then I called to cancel the decoder that I was using because they said I must tell the installer to do so and I also did tell the installer, so they can only deduct for the new decoder only but guess what last month I paid R635 when I asked they came with stories then I bear with them and they said this month I will pay R499,yesterday they deducted R1281,13 when I ask them they now sayisaying I didn't cancel the other decoder but I did so I want them to reverse my money they are refusing