Telkom destroyed my credit record

I had an account with Telkom and I cancelled it last year. They carried on charging after confirmation of my account being cancelled. I continued to fight this with accounts and never got anywhere. No-one could give me correct information. Each month I received a new bill that just got higher and higher. Being that I cancelled my account, I was not going to pay this. I then received a call from Blake and associates as Telkom handed my account over. They said I owed R677 and if I don't pay I would be black listed, which I just paid as I did not want a bad credit record. MONTHS later after being phoned by both Telkom and Blake and Associates and being told everything was sorted as well as receiving a bill from Telkom with a R0 amount, I find out I am black listed. I can not apply for ANYTHING because of this. I am beyond furious.