Pathetic service and threatening legal action for them not doing their work

I have cancelled my contract with Altech Netstar in Febuary 2018, I have called their call centre numerous times querying why are they still billing. To date I do not get an answer and now they threaten to black list me. I have been with this company for 7 years and now that I cancelled my contract they threaten to black list me and take legal action. Maybe they should stop wasting the time sending messages and calling and start doing their jobs.