Still debiting my account 4 months after migration

Last year November i migrated my cell number to MTN taking out a new contract with them. I had been with Vodacom for more than 15 years, however bad reception made it necessary to move. My contract had expired but they continued to debit my account. After many hours on the phone and speaking to a number of people the problem has not been addressed, this is ridiculous i wonder how many others are unknowingly been billed ? What do i do now ? - Pat