Cancelled yet still being invoiced one year on

I cancelled my Telkom account in December and they are still invoicing me every month. After a numerous amount of calls and promises by call centre staff the account is still getting invoiced. As I am not paying any of these account I now have a reduced credit rating as it reflects on my credit record that I am a bad payer. I have about 10 reference numbers that are all just as useless as Telkom.