Hathway Cable & Datacom — connection cancellation and blocking phone calls daily

I was using Hathway broadband approximately 7-8 months ago. Ever since I stopped using it, I get daily phone calls from your call centre. It is very overwhelming to receive all these phone calls when I have explained my case clearly to each one of them. The customer care agents have continuously promised me of putting my phone number under “Do not disturb (DND)”. Nothing has been done so far.
Today I tried to reach out to the helpline where they completely refused to help me or even transfer my call to the concerned department.
In case you want my case to be further justified, refer to the official email I have dropped on helpdesk, where I have shared my details.
It is highly disturbing for an individual to attend as good as 2-3 phone calls everyday. Kindly look into the matter asap otherwise I will have to take some legal action. Please make it stop.