Wrong debit dates

I'm a member of Virgin Active and I don't understand what's going on in their accounts department. For the past 3-4 months they decided to debit my account on the wrong dates, i.e the 1st and the 15th of the month. When I took out membership with Virgin Active I asked them to debit my account on the 25th of every month, that was done for a short period of time and then they just decided to debit on the dates that I've mentioned, when there's not enough funds in my account. I incurred bank charges as a result, I've tried several times to rectify that but nothing was done. At some stage when I phoned them they asked me to write an email which I did but still no one contacted me. Last week I got a call to inform me that my account is in arrears and again I told the person who called me about my problem and emails that I wrote them, I also informed her that I'm disgusted with their service and would like to terminate my membership with immediate effect. She promised to escalate the matter and that was it. This evening they debited my account AGAIN!!!!I'm about to lose my mind!!!