Wrongfully blacklisted

I made an order at Justine around October/September 2014 but this could not be delivered to me because of the post office strike that time. The order was the returned at Justine, 4 months later when I wanted to place another order was told that my account was owing about +R 600. 00 of which they later rectified and reversed the charge but now the problem is that recently this reflects on my credit record and aperently the account was handed over because of the +R600.00 that I didn't even recieve the orders for and now I am having trouble in fixing this problem Justine is not willing to fix it as if it was my fault that post office went on strike. I am realy left stranded and I am still trying to figure out how am I going to get out of this. I have been blacklisted for something I did not do.