No contract, yet the still harass me for payment

I added a car to my Auto & General insurance policy several years back (prior June 2014). Part of the cover was the fitment of a Tracker device, the cost of which was covered by A&G, including the monthly subscriptions. At no time did I ever sign any agreement with Tracker directly, nor did I ever pay them anything. In September 2017, I gave 1 month notice of cancellation of the car insurance with A&G and confirmed with them that as the Tracker device had been installed for more than 3 years, no cancellation fee was necessary. In November 2017, I started receiving SMS's from Tracker stating my account was overdue. I called them and explained that I had never signed any agreement with them directly. The agreement was between A&G and Tracker, so it was A&G's responsibility to give notice to Tracker. The consultant seemed to understand and on 23 November 2017, I received a letter from Tracker acknowledging the cancellation, albeit effective 31/12/2017. On 30 November 2017, I received advice of a debit order by Tracker from my bank. I contacted my bank and explained I had never signed any contract with Tracker, nor did I sign a debit order authority with them, so the bank was to reverse the transaction. The bank did so. I sent an e-mail to ********** advising them of the reversal as well as reminding them it was A&G's responsibility to cancel the agreement with them. I received an automated response stating the 'mailbox is unattended'. Really, your customer service e-mail box is unattended? I again called Tracker and explained the situation. In January 2018 I received SMS's advising me my Tracker account was overdue. I ignored them as I wasn't wasting any more time with people who clearly do not know what they are doing. On 18 February 2018 I received a SMS saying my debt had been handed over to collection agency "NBA". I again called Tracker and explained the situation to David Motlhomi, He said he would have to escalate the matter. I have now received 5 phone calls from NBA in the last 2 days, despite me explaining the situation to the first caller. This is harassment. What does it take to get the message through to these people?