You cant get out of expired MTN contracts

I was a loyal MTN customer for about 15 years. Due to decline in service quality I decided to end my contract with them. I have been trying to cancel my contract now from January 2026. It expired in October 2025 already but they keep deducting every month. I send a letter to inform them of cancelation of contract to at least give formal notice and still they pushed a debit order through every month, no matter how many times I call the number and speak to the multitude of people I get passed on to. Please help!