Asked to pay for an old decorder that I cancelled 3 years ago and threw away

In May, I requested my account balance via WhatsApp. DSTV activated an old decoder (which I had discarded 3 years ago) without my knowledge or consent. In June, I paid my usual subscription. DSTV then informed me that I short-paid, because they billed for both decoders — including the one I no longer have. When I called to explain, they insisted I still have to pay for the inactive/old decoder to clear my account. I asked to cancel my account, but they refused, citing the outstanding balance — even though DSTV is a prepaid service. I cannot be expected to pay for a service/device I did not use. As a prepaid service, DSTV should not be charging me in arrears for services not agreed to. Refusing to cancel an account due to an error they caused is ********* or even ********. DSTV you are a SKELEM.