Cancelled within a day, but still getting billed

I ordered a tablet and received it. The quality were very bad and I cancelled within a day. Vodacom sent Ram couriers to collect the tablet. I manage to get through to one helpful person that could tell me that he can see the stock was received and book in on the 1 September 2017 but I'm still receiving bills. On a letter they are saying that if I do not pay my account within 7 Days I will receive a bad credit rating. I cant seem to get through to a live person to help me to cancel the contract but only a computer requesting me to input a contract no or cell no that I do not have.