Yes - you can't. Absolutely impossible. Not even VM can do it. It is linked inextricably to their system and will always be valid for logging in to My Virginmedia. But you can let it go dormant and eventually after 150 days of complete non-access VM will deactivate it for e-mail purposes.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.