BAD GAME credit card system

some time is April/May I bought a calculator with game credit card and 3 weeks later i went to the store to pay off the credit card, they gave me a settlement amount and I paid it off. 2/3 months later they started sending me sms every week to remind me that I owe them R134, I called and complained. They agreed that they made a mistake however they they continued to send me the sms's, once i again I called spending over R50 every time i call through airtime due to waiting on the line, they promised they will erase it and also call me back to confirm but never did. This morning 14th Sep got yet another sms this time the amount has went up by R37 to R171. Can anyone please help me with this please - I have worked hard to ensure that I have a good credit record to be wasted over couple of rands.