Netstar still billing after cancellation over a year ago.

I've traded in my car to the dealership in 2023 Aug-Sep which had a tracking device that I paid for in 2021. The car was no longer in my name however, I still received an account from Netstar in June 2024 with an outstanding amount of R140. So, I called to inquiry only to be told I was still contracted with Netstar even though the vehicle is in the new owner's name, but I was promised this was an error and it will be rectified but I shouldn't be liable for anything. Months passed by I'm still receiving notices that I'm in arrears. This company is a rip-off, how can you tell a customer they don't owe anything but you still sending bills monthly? The departments are not talking to each within this organization.