Constant Overbilling

I keep being over billed by multichoice. On the 15th of March my decoder gave me an E30 error I took it back to the store an installer came to activate the new one on the 17th I called them to transfer services to my surprise instead of paying R399 as usual I was billed R580 because the faulty decoder was never cancelled. I just Checked how much I'm expected to pay next month yet again instead of R399 its already billing me R50 extra. I called the call center the agents are tongue tied & unable to bring down these ridiculous over billings. Who does one have to talk to to get this fixed, I've called, I've sent emails, WhatsApps no luck.