Can you confirm that the address is not linked to a current VM billing/broadband account? i.e. you are not a VM customer?
As you may know, the mailboxes should have been closed AND deleted automatically 90 days after you finished being a customer (1 year for virgin.net IIRC), according to VM's own Terms and Conditions. But the process fails all too often and leaves the former customer vulnerable to hacking and theft of personal and financial details (IMO, someone should definitely claim for damages from VM or complain to ICO!).
Only VM staff can raise a ticket to get the orphaned mailboxes deleted manually. The friendly Forum Team will visit this thread in the next few days and then take your personal details by private message.
When they say it is done, you can verify it is gone by running the email address(es) throughhttps://mailtester.com.
In the meantime, if you still have access via an email client (try Thunderbird - using IMAP process which synchronises with the mailbox stored on the VM server), delete everything from all the folders to reduce the information available to hackers.