HORRIBLE SERVICE PROVIDER WITH NO CUSTOMER SERVICE

I have requested cancellation in November 2025 explainedclearly to the agent i spoke to that i have a better service provider and because of their lack of service there is no need to continue, i am out of contract, they told me to pay around R40 cancellation which i did in december and still im being haunted by their debit order requests on my bank and tons of calls to my phone and spamming my email with requests for payment and everytime i raise the issue im told i need to speak to an agent for cancellation providing a written notice doesnt work. how do i do that when i not only spoke to someone but also on 2 occasions responded to their email requesting immediate termination of service