But from your description, it sounds as if it is the primary email address on the account, and therefore can't be deleted. It is part of the single sign-on system. The mailbox is supposed to go dormant after 150 days of non-use, but I haven't personally seen any evidence of this.
So all you can do is make sure you use a secure password for billing account access, and change it regularly. After you are sure you have changed any other places that use the address, you can backup what you want from the mailbox, then empty the address of any data - emails and contacts, etc. Turn off all filters and forwarding. Then don't access it, don't think about it!
The Forum Team (VM employees) can confirm this by looking at your account set-up behind the scenes. I have also noticed the VIPs say that blueyonder aliases can be deleted now, so perhaps that can reduce the amount of worry you have over hacking and spam?
Thanks for your post, I'm very sorry to hear that you've been having email issues. @MissPasko, was spot on with her advice. We can get this mailbox closed for you, so long as it it not the primary email to an active Virgin Media account. If it is a sub-account, you should be able to remove this mailbox from within the settings of the primary email account but also do make sure that you do not have any accounts or services that are dependent on this email as a username or log in credentials, because you may not be able to sign in or update those details once the mailbox has been removed.