Can't cancel contract

It is impossible to cancel my contract with MTN despite already being past the contract period. The first rep I spoke with claimed that I must give them 30 days notice, even if the contract period has already ended. The next rep hung up on me. I went into an MTN shop to interact with a consultant in person. She couldn't believe what I was telling her and tried phoning the call centre herself in my presence. After they hung up on her as well, she said she would change me to a pay as you go plan and gave me a reference number which she showed me on her screen. When I called the call centre a few days later to see whay this didn't happen, I was told that the "in-store consultants don't know anything about cancellations" and that my reference number was meaningless.

This is all despite having an account with MTN for many years and having just done a few upgrades with them for contracts on the same account.