WORST SERVICE EVER!!!!

I opened a data Sim contract for mtn with rewardsco.com which I was sold 60gig instead of 20 gig per month. I asked them to cancel my account after they found out that their sales person made a mistake over the phone. They told me that it has been canceled but I'm still getting invoices and debit orders from my account which has affected my credit score badly. I don't even have that data Sim anymore since I canceled it. I tried calling mtn and they couldn't help. I called rewardsco and I have been taken from pillar to post and on the 12th their consultant "sbo" kept me on hold until they closed their offices which I tried calling back again but it was after their office hours. I am so angry because I'm still getting invoices and debit orders for a service which is supposed to be canceled by now.