Cool Ideas is a ****!

Good Day I hope you can assist me. I am in desperate need of help.I had a month-to-month contract with Cool Ideas from April to July 2020 but the internet was very slow so I gave them a 1 month notice as per their contract. The contract was for the fiber internet in my home. After I canceled them I tried to find another service provider (Vodacom) but they could not connect me as Cool Ideas had not vacated the line. I called Cool Ideas a number of times to ask them to vacate the line. They will say it was done but then it was not done because my internet was still working. I was so frustrated during this time as I was working from home and needed fast internet. I eventually cancelled my contract with Vodacom and tried to go back to Cool Ideas as they were refusing to vacate the line. I had spent so much money by that time buying mobile data bundles. Now I have received the attached invoice from Cool Ideas saying I owe them money for the period when they failed to disconnect the line. They are threatening to blacklist me if I do not pay. I do not understand how can that be legal? Isn't a month-to-month contract meant to avoid such situations?