billed incorrectly

when I took my cellphone contract at Autopage hillcrest last year I was assisted by a guy called Brandon. We did everything and then I requested him to make a Limit on my contract to be R500 and nothing more. The first 2 months I was billed incorrectly and he sorted it out. until yesterday the 15th of may when I was billed R1333.94. I almost had a heart attack as to what I owe to this. only to find out that Brandon never did activate a limit on my contract. as a result I refuse to pay for Autopages employee faults. not only that I was told I still owe R4000 plus for June. I am not going to pay a cent for this. I clearly asked for a limit and its not my fault Brandon did not activate it. They must rectify this mistake and let me carry on with my normal bill.