Unauthorised Deduction of my Bank Account.

I was on a month-to-month contract with VIVA Gym in Rosebank. I canceled my membership and gave them a months notice. They acknowledged my cancelation through SMS and email. However, they deduced a month's fees again from my bank account, even though you pay a month in advance. This has left a sour taste in my mouth, overall the gym and service are of a high quality but this is simply unacceptable. I demand that these fees be reimbursed to me, or I will take this matter further.