Angry, disappointed and frustrated with Identity account department

I am so disappointed with identity account department i have been charged for the 12 months interest that i never signed for, and they are calling me non stop, i never missed any payment with them, when i asked them who authorised this plan they can't answer i sent emails with no luck, i have to return some of the items i bought out of frustration and still they still say my installment is the same, how come i pay R470 for the balance of R1335 of which i am on six months plan. This new plan was added in August statement and i told them even then i won't pay that money now they are telling me i have an overdue payment of R470, i asked them to review this. I feel like they don't want to. This thing will affect my credit score.