MTN Ambush

There was a self-service purchase for R8 000 that was done on my number 078 413 ***X without my knowledge or acknowledgment on May 28. I have called the customer service department and several service requests have been logged but I'm not getting any help. I want this service purchase canceled and any amounts incorrectly billed by MTN SA to me to be credited. The contract on this number ended in August and I want it canceled to prepaid. The device in question was purchased in Johannesburg and I live in Durban.