Wrong billing and unprofessional handling

I was billed R25 monthly for the magazine without my consent, I notified that cashier on the 29th of September 2014 when I paid and she cancelled it on the system then said I'll never happen again. The following month the same problem occurred when I was I was going to pay the balance, she claimed to sort it out and told me not to worry because I had finished paying the debt. The slip reflected that the R25 was outstanding but she'll sort it out not to worry, then after few months I get a statement about me owing them more than R1000, I even receive calls from the cashiers harassing me about my \supposed"credit which I kept telling them that I don't have any credit with them and they should stop calling. Last month I called the store manager complaining about this and she promised to sort it out and she shifted blame on the head office. This month again I get a statement saying I owe them over R2000. I'm ****ed off about this because it's over 2 years that I paid the balance but they still harass me"