Unfortunately, I suspect there's nothing you can do. If you can provide a trail from that email to your current one, maybe the ff.net host will believe that it's your fic, and remove it. Otherwise, it'd be too easy to pick any apparently abandoned set of stories and say "I wrote those and I lost my email; please remove them." (Maybe if he sends out a test email to the addresss, and it bounces, that's considered confirmation.)
It looks like the contact email is support@fanfiction.com. (It is non-obvious; it's buried in the TOS under how you report copyright infringement. Currently, it's also on the home page as the address to give feedback for one of their new features, so I assume it's the general contact address.)