paying for maths coded CD that cannot be used for its purpose

I was called to the school called Davey high school and was told that the child is struggling with mathematics. The guys from Maths Ace explain how it works .The child was doing grade 11 then. i was paying monthly R 185.00. I paid from June 2013 until 2015.The issue I received a Disk which had a code .i tried to call them but were not picking the phone. This went on until my child finish grade twelve. I decided to change the account and that when the call to check why I am not paying .I complaint to them about the CD which was not used because of lack of their code. They apologised and still never gave the code. however agreed that now that they have been taking my money it is fine but will not continue paying .i was surprised to receive the call from the lawyers who demanded that I pay.as we speak they have blacklisted me. the problem is that no one hear that yes I agree to buy the CD for the child but the child never used it because they never provided the code to use it. Now my name is unfairly blacklist .I need help on the matter because at least they keep the money and not blacklist me with out fault