Contract cancelled 4 months ago, but MTN kept debit order

I am deaf and had phone contract for ex-wife and she had gone to South Korea for a year contract. I asked MTN Thibault Square office to convert to prepaid on 7 August 2020. They did it, Reference number is ***. But MTN still debit order every month on 1st Sept, 1st Oct, 1st Nov, and today. I kept going to the office to make sure thy stop debit order. They struggled to get hold of account department because they work from home. I went to my standard bank to stop the debit order, but they said, they can't do it. So what must i do now? It feels like both don't care and MTN ****** me for 4 months..