Check if the address is linked to your broadband billing account - sign in and it would show on the My Profile page (scroll down to my services), or the Manage Accounts tab. See the photos below. If it is the primary address, neither you nor VM can delete it, so keep it secure with a good password. If it is a secondary address, that should be possible from the manage accounts tab. First backup anything you want, then delete all the data from the address, then delete it.
If those options are not available, then wait for the Forum Team who can request a manual deletion. They will take your details by private message to pass security.
You can test it is completely gone from the VM servers by running the address through the resource at https://mailtester.com. Best wishes.
Thanks for checking - they will certainly be able to help delete this orphan address and should reach this thread in a day or so. Can you just let them know if it is showing as valid in mailtester? Not the address, just the result.