Cartrack ,a total *******,why cancellation fee

Good day,i was a client of cartrack. I sold my car and notify them.I received an email back stating that i am supposed to pay a cancellation fee. Now this puzzled me,because i bought the car from Autopedigree and was referred to cartrack, so i trade the car in,and was nolonger the owner of it,thats why i send them an email to cancel it,Dispite my email they were still billing me,and sending emails that i have to pay about +-R1200 to cancel the tracker. Now i receive a letter that they have blacklisted me? Why they havent explain this to me when they installed the device in that car,of which the installation was free. Now they want me to pay for a device that is in a car that i've sold,of which they just reactivate it to the new owner, so i have to pay cancellation fee,so that they can "reinstall" the tracker in the car for the new owner? I will rather pay a lawyer,to setle this, than to be ripped of like this.