Account query

I have been struggling for months now with gym company, the problem started when they said they did not debit my account on May this year, but on my bank statement it shows that the funds where debited, i emailed them my statement but they still claim they did not get the funds and they charged me arrears on the account, i gave up because we where not going anywhere and agreed to pay the overdue because i don't want my credit status to be bad. I paid the overdue last month and they said my gym account will be up to date within three days, but this months they debited more again and i called them they told me they have a system error and they will call me back but never did. I feel like i am abused by the kind of service gym company is giving me, i am afraid if i cancel their debit order on my account i will be black listed.