i will be blacklisted if i dont act soon

on the 28 of february 2015 i was at GOLDENWALK one of the consultants from body lab told me that they were doing a promotion and i will gym for free for a month and after a month if i dont like the gym i can cancel it for free at no cost,he asked for my bank details i asked him about 3 times if i wont get into trouble if i signed he said no the are no hidden agendas its only a voucher and i have to sign and they are only following procedures,after a few days i went to them to cancel it the told me to pay 30% i told them im not working because i just completed my matric last year THOKO told me that the is nothing the can do if i cant pay because i signed but they said its a voucher not a contract now 3 months down the line i get a later from their lawyers BENNETH AND ASSOCIATES saying i owe the 10832.83 HOW? because i never used their gym even for that free month now they are threatning to blacklist me BODY LAB MUST CLOSE DOWN because they are destroying our lives how will i pay im unemployed onl completed my matric last year please help