Unsolicited Debit Order

I joined Virgin Active gym and agreed on a price and membership with the sales consultant, but I specified to the consultant that I do NOT want Virgin active to put a debit order on my account. The consultant said that this is not a problem and that I can pay a lump sum (say 50% of my membership fees) and the rest of the lump sum I could pay when it is due. I then proceeded to fill out the contract and it asked for my banking details. I AGAIN told the consultant that I ABSOLUTELY DO NOT want a debit order, the consultant assured me that I should just fill out the form and there would be no debit order. This was a huge mistake on my behalf as I just found out that they made me sign a debit order contract. I decided not to return to the gym as the time for which I had already payed elapsed , only to find out I was deceived by the sales consultant in terms of the Debit order. Now Virgin active decided to put a debit order on my account to try and claim the rest of my membership fees. The account to which the details I gave them, I do not keep any money in and when they tried to take a Debit order, it bounced. Now I am being held hostage by the likes of Virgin Active (which I specifically tried to avoid by not getting a debit order in the first place), they would not reverse the debit order until I pay them the amount that bounced plus a cancellation fee.