Pathetic customer service, bridge of contract

My wife took out a new contract, with clearly stating that she wants a closed contract, mention that if her data/airtime is finished, she needs to buy new data/airtime. Well, about 2 months into the contract she started getting bills of R2000, R3000 a month!!! She has stop her debit order and only pay over the contract amount, but now every singly month they cut off her phone and have even threatened that they are going to blacklist her. We have been trying to sort this out for the past 5 months. She even wrote about 3 letters telling them to cancel the contract. MTN this is PATHETIC!!!!!! THE STORE MANAGER PROMISE AND PROMISE AND NOTHING HAPPENS!!!!!! CUSTOMER SERVICE IS NON EXISTENT. My wife is losing out on business due to MTN cutting off her phone and it takes weeks before they put it on again. We would really like this to be sorted out immediately!!!!!