paying for a service i do'nt get at all

i took an insurance with the package of tracker device the intallation was done wrong and ended up removing it , i then cancelled the whole package including the tracker device but now they are billing me with the service i do'nt get they are refusing to cancel my account and they took my account to nudebt collectors and i am forced to pay R733.22 which i payed with the hope that this account will be closed but to my suprise they are still continueing to bill me, i asked them why they do this because i do'nt have their device in my car. please assist me in this matter its making me sick.