Debit order not cancelled

I paid Many thousands of rands to terminate my cell phone contract last month but for the second time MTN still pushes their debit order through my bank account & yet my number is now a prepaid line. My email from last month was ignored. Can you guys get your house in order, surely you've had enough of my money after all these years!! The process of reversing debit orders is cumbersome & I can't be doing it every month just to accommodate your inefficiencies.