Cancelled but I am STILL getting billed FOR A CAR I NO LONGER OWN

I have no idea how to handle this situation anymore.

I have cancelled my matrix tracker. I received an invoice and complained here since contacting matrix directly does not help. I spoke to a very nice lady that assisted me. On the 12th of July I received confirmation that my account has been cancelled and closed, so now why am I still being billed for August?

I am fed up now! This is effecting my credit score badly as it show I have an account in arrears when I actually do not! I don't owe matrix anything as confirmed telephonically.