they debit my account as they pleased

Im a virgin active member, I asked them to debit my account on the 15th of every month, they used to debit on month end.so last month they debited my account month end and on the 15th, when I phoned them they said my account is on credit they are going to refund me.i phoned they after the 15th.as I expected my refund another debit order went through month end. Which is they've debited my account three times in one month.this causes my bank account to be overdrawn and I have to encur charges.moreover the calls that I have to mak to them are charging me.im so furious.