Villanous actions by Vodacom

I terminated my contract with Vodacom a few years ago. Now I find that they have blacklisted me, apparently for one payment not made (a ridiculous amount of around R260). Even though I have a final statement from them showing that I owe them nil. I want to know how this happened. And how did this happen without them informing me that "I still owe them one payment"? And how can they blacklist me without even warning me about this villainous action that they took? On another point, Why are the totally unprofessional staff at the V&A waterfront staff allowed to be rude to customers like myself? Is there a manager in that shop? It seems they are only polite when they want you to buy something, but when you need them to fix the errors they create through their incompetence, almost no one is available to help. There was only one person that was ever helpful to me in that circus of a shop.