Worst experience and bad treatment

I had worst experience ever with Netstar, firstly I they installed the unit which was forever faulty and lastly my contract expired December 2019 which I informed them that I won't renewed it but yet they went ahead on January billing me and they even handed my account to a collection agency, when I called them to enquire about it they apologies saying they will fix the error but on a daily basis the collection agency send me threatening sms saying they represent Netstar and saying that blacklist me,I regret ever joining Netstar