Still getting billed despite multiple cancellations

I've never used this service, and barely remember why I even had it activated. I tried to cancel in May this year, was told I was successful. They were happy to inform me that my query was resolved. Then June and July they happily continued deducting their service fee. I brought it up again with their support guys, and was told it was being seen to. We're now in October, and up to now they have continued debiting my account. Relatively small amounts perhaps, but I'm an entrepreneur now so there isn't always money in my account. So now I have to pay for bounced debit orders on top of their service fee for a service I never wanted or used in the first place. Can't I escalate this to some or other regulatory body please? Someone help? I want to claim my money back.