Netstar attempting to highjack our funds

Netstar keeps charging me for services not rendered , when i sold my car i cancelled the tracking services and requested a cancellation , and have mails to prove it , but they keep sending me invoices and threatening to list me , I've contacted them multiple time and they keep saying they'll get back to me but never do, they just repeatedly send threats ,where and how do i report them. I will be in a casket before i pay them a dime. my reference with them is ***