Unauthorized billing to my account

Virgin Active called me in the beginning of the month, to say that my account is late. So I told them yes I know because I have went in to their branch twice changing my debit order details. I helped them to correct their mistake by giving them my new details again. They asked that if they can debit the full amount and I said no, and specified the amount. They did debit that amount, and now 15 dais later they decide they want to debit more money out of my account? I never said they can debit money out of my account in the middle of the month. Always on the first of every month. I want my money back!!! Otherwise that is just acting like a ******** in my eyes.