Virgin Active - doesn’t cancel mean cancel?

I am having a terrible experience with Virgin Active gym. I was not enjoying the gym, so I cancelled my contract. I paid the cancellation fee in full, which was quite a hefty sum, although in the contract it stated that they would only charge a minimal cancellation fee. They now keep trying to debit my account and sending me SMSs that I have not paid them. Surely your service should be good enough not to resort to these tactics to make money!