Very Upset

So I had a phone call from Shaw Academy because they saw me looking at some free online courses they had, I saw this add on Facebook and decided to have a look at what they had, I never finished putting in my details as they were asking for my banking details witch I find very suspect. I then received a phone call from a women who informed me that our phone calls are being recorded for security purposes, I listened to what she had to say. I was informed that under no circumstances will they deduct money from my account unless I choose to carry on with the course after my 4 weeks free has been done. During that 4 weeks I only attended one class online as I didn't have the time to do so. I see every now and again I get some emails from them but I ignore it as I am job hunting at the moment so I don't have the time to read it. Today I realized that they are deducting money from my account and that it's a recurring deduction, my problem with this is, I never gave them the right to access my account, I was told that they need the account details in order to process my details for my free 4 week course, yet they take money from my account that I did not ok. I am very upset about this because I only had R150.00 in my account that was given to me by my mother to buy my son a birthday percent for his birthday on Sunday, can you imagine how I felt when standing at the shop with my son's gift in my hand and my card says declined (when I knew I had the money in my account to buy it)? Not only is my heart broken because I can't buy my son his 5th birthday percent but I am upset at the fact that they didn't have my permission to deduct from my account... This to me is utterly disgusting to do so cause surely you need someone's otherization to take money from their account? Am I wrong, Am I not understanding this correctly?