O2 is charging be for an extinct account

Hi,

When my phone accounts were migrated to O2, they began to charge me for a sim that had been lost (along with the phone) some seven or so years earlier and which had been cancelled by VM. It was so lost I don't have the full number and do not remember what phone it was on. The bank cancelled the direct debit at the end of August as they tell me I should not be paying for a service that I am not receiving. 

I feel I have exhausted all routes to resolving this issue: I have tried to communicate via the ChatBot endless times. The bot does not recognise my topic. Offers to connect on WhatsApp have been accepted but I never get the promised link. I  have phoned and managed to get through to human beings twice. Neither have been able to resolve the issue, with the clearest explanation being that as I don't know the number or the phone details, they are unable to verify that I own the account. My explanation that I no longer own the account but am paying for it seems to complicate the conversation. I am directed to a shop.

At the shop, the assistant is able to identify an account but is confused as it doesn't have my personal email on it. It has a default O2 email attached to it.  When he phoned O2, they advised I should contact them by phone. See above. 

The shop assistant further muddied the waters by telling me I needed a sim in order to cancel the account and gave me one, with the 'old' number. I have not used it. 

I have sent three physical letters and three emails to the complaints email but I have received no answer. I am getting emails to tell me the service is being restricted, but as I point out in letters/emails, this is no hardship as I wasn't using the service anyway. They want to cancel the contract but seem not to realise I no longer have a contract with them. I want their emails to stop, along with the threat to report me to the credit agency, and I want a refund. How can I bounce them into a response other than a Small Claims referral or a letter to the Ombudsman?

Any advice gratefully received.