Continues phone calls on Outstanding Account

I continuously receive phone calls from Altech Netstar that my account is outstanding even though the vehicle was sold 6 months ago.

I send them an email to cancel the tracking unit the day I sold the vehicle. I did not receive a response that this was in fact done and I did not follow up on this again. 3 months later the phone calls started…

I then again send an email to cancel the unit and this time I phoned to ensure that this is in fact cancelled. I also paid an additional 4 months that they claimed was outstanding after which they send me a confirmation email that the account has been closed but to date I still receive phone calls that the account is outstanding.