Very old ntlworld.com email still working

I left Virgin Media probably over 10 years ago, and have no account with them. I have an nlworld.com email address that is still working - in fact this forum' registration didn't recognise my mail.com address so I used the old ntlworld.com address to verify my profile!

I have no control over this ntlworld.com account any more as I haven't been a diamond media / ntl / virgin customer for years, but it keeps receiving emails (all junk now) and I can send emails from it too. I think this is a security risk as I can't change passwords if it gets hacked.

How on earth can I close this  email account?