Abusive and Refusal to listen

We had a business account with Netstar. After months of struggling to change online login details and banking details we started getting calls from Creditworx debt collection. We explained that as we could not use the service or get Netstar to change banking details we were cancelling. As recommended by The proof of payment for the outstanding amount and cancellation notice were sent to Netstar and Creditworx on 25 November 2019. In February we were asked to send the proof and cancellation again. Now, Creditworx is still hounding the previous owner for the money that was already paid in November. They refuse to understand that they are calling for a matter that was resolved 6 months ago. The previous owner is 75 and they are being abusive. This is disgusting!