Abysmal Supersonic Service

Since being taken over by MTN Supersonic, their service and response to requests have been abysmal. As a result, I have canceled my business service with them and am trying to cancel my home service, but once again, they have not canceled my service and released my Fibre line for my new ISP. I paid for their service until the end of January but now I need to connect to my new ISP. How can they continue to hold my Fibre line legally?