Tracker Poor Service

I sold my car 3 years ago and while doing so cancelled my tracker membership on that car. I am now being contacted by debt collectors for non-payment. I battled to prove to them that i did in fact close my account. In the interest of resolving the matter i settled the oustanding amount and sent them proof of payment. Despite this i am still receiving call from debt collectors with legal threats which no doubt has also impacted my credit score. I am still a member of tracker on my current car if this isnt sorted they will lose this business too.