Constant harassment for a non existent debt

I terminated membership of Virgin Active in March 2023. After one year, I I get a message from MBD that my account has been handed over. I queried VA and they had billed me for one extra month in error. After much back and forth, I received an email from VA stating that my membership was terminated and that I had a zero balance with the relevant spreadsheet enclosed. I must add that I never owed any money and that my account was always paid via a debit order.I communicated all this information to MBD but despite this I continue to be harassed by their representatives via SMS, email and telephone. I have re-sent all of the info three times to no avail. I still receive threatening emails, the latest on 4 May 2024. If this does not cease, I will be forced to report them to the Legal council.