Being harassed by Netstar

My tracker was installed by my insurance company, I had to have a tracker in order to have comprehensive insurance. I have never payed Netstar, I paid my insurance company and they paid Netstar. My vehicle was written off in April 2014 and in December 2014 there suddenly was a debit order deducted from account from Netstar. I am not liable for the money owed since it had to be cancelled by my insurance. I have explained this numerous times to my insurance company and Netstar but a collection agency keeps on phoning me! The contract has been cancelled on more than one occasion but I was advised that it's still active. I am so fed up with this!! This has been going on for almost a year now!