Cancelled- Stilll billed threatened to be blacklisted over R396.68-Furious

I am so ****ed off. I cancelled my month to month contract, 3 months down the line i receive a bill for R396.68- I have emailed them back to no avail and response. Today received an email stating i will be blacklisted after 20 Days . Hell alone knows why do i owe them money when i received an email stating they received my cancellation on 1 August 2020.Service is up to **** as no one answers the phones and they keep on advising that you need to WhatsApp them. I am at wits end and furious!!!!