Being held liable for purchases that I did NOT make!!

********** airtime purchases were made on my Sheet Street account. I had been asked to submit an affidavit, which I had done, they then insisted that I had made the purchases from my Mr. Price Mobile sim card. They had concluded the investigation on these "findings." I then demanded for valid proof to support their statement, only to discover that the airtime purchased on the sim card and the airtime purchases on the statement DO NOT correspond, when I emailed them to explain further, 2 weeks later and no response!!!! Now I am held liable for purchases that I did not make!!!! They do not value their customers.