Refusing a Cellphone Purchase

This morning i visited Truworths Springs Mall, to buy a cheap Cellphone on my account, my old cellphone was snatched from me yesterday, when the lady at the couner processed the sale she informed me that i cant buy a cellphone on my account, HOW EMBARRASING this was in front of a shop full of customers in the queue behind me, when i asked why i cant buy because i had more credit available than the cellphone cost, i was told the SYSTEM is blocked, i continuously receive sms's to go and shop and thank you for me being a loyal customer, and i was even granted a higher credit limit and now this ??? Why am i being treated like this, and over a Christmas Period ?? All i wanted was just to buy myself a new Phone as mine was stollen from me. I asked the lady to phone the head office and she informed me that phoning will not help as the system wont allow it, this is such disgusting treatment as my account is up to date with no outstanding balance, why send me sms's and e mails to come and shop and increase my limits just to be embarrased in front of the whole shop and other customers. I am SO DISSAPOINTED !!