Contract Cancelled but still receiving the bill

In May 2015 my mother who is an elder went to MTN branch in Thohoyandou to cancel her contract which in now over 24 months the person who assisted her told her that the fasted way to do it will be to call the head office and cancel it rather than filling the cancellation/ migration forms in the store she then assisted her to call the head office and it was confirm that her contract was cancelled and she was given the following ref (***) to her surprise beginning of June she receive the air time as if her contract was still active I then go and enquire in one of the MTN stores and they confirm that her contract was still active when I ask why because she even have a reference they couldn't explain why . Can someone explain why such an old woman must run around just to cancel a simple contract? It looks like that's how MTN operate because I'm also sitting with exactly the same issue I've cancelled my contract at Irene MTN store also in May 15 and in June I still received free air time and when I went back to the store they request be to fill the form again but there was no explanation of what happen. MTN is indirectly forcing people to continue with their contract .