Is the address linked to your VM broadband account? If it is a secondary address (go to the Manage Accounts tab in My Virgin Media), then you can delete from that page. I would recommend first deleting all the data (emails and contacts) via webmail. Afterwards, check the mailbox has been deleted from the VM server by running the address through https://mailtester.com If still valid, come back to the forum and ask for a manual deletion.
If it is your primary address, it can't be deleted. You can change the username of your account to be a non-VM email address, and stop accessing the mailbox. After 150 days it is supposed to become dormant.
As you may know, the mailbox should have been closed AND deleted automatically 90 days after you finished being a customer (or 1 year for virgin.net addresses 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. They are very busy with a major issue affecting customer email/mailboxes so please be patient.
Just be sure you haven't got any other online accounts/services which need this email address as a username/sign on. 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.
When they say it is done, you can verify it is gone by running the email address(es) throughhttps://mailtester.com.