Edgar's charging me for services I did not ask and don't even benefit

I went to JMT brakpan tsakane shop 40, tsakane shopping center last week and as I enter the store I was stopped by an agent asking if I have an account with them, he is doing a promotion I will get free airtime he is going to give me a vodacom sim card all he needs is my card. And I agreed the next thing I find out a week later than money will be deducted from my account every month and I can't cancel and worst part is I did not get that airtime. I'm being charged for airtime that I'm not even receiving