Demand of Payment to closed account

These people are very worse they seem to be desperate to make their next penny by demanding outstanding payments of closed accounts seems like they are in the business of cheating people and I think they also get paid by these companies to do this ,I have a paid up letter that I sent to them 3 to 4 times that proves that I finished my Cell C account yet they continue saying that I owe Cell C , they are running some kinda **** together with Cell C.