CreditWorx Harrassment from ZoneFitness Account

I recently joined Zone Fitness gym and an error was made with my banking details on their part and no debits were made to my account. They said this was my fault since their banking details were the same as those provided by my bankers.

As payment had not been made for three months, I paid the full membership fee due so as to close the account. I do not know why payment did not go through as it did not constitute a bounced debit order so I shut the account down to avoid any credit implications. My account has however been handed over to creditworx who call me day and night ( I have literally just received a call from them at 8pm which I believe is unacceptable) . I dealt directly with the Debtors department and paid the amount they said was due but I am still being incessantly called /borderline harassed by these people.

I no longer know what to do because I have paid everything to close my account. This is extremely frustrating and infuriating as I have paid.