Report them to their ISP. Or to the email provider if one of those free ones. Which may be better as they can then stop their email (bit of a bummer if you are attached to it) or perhaps even report them to their ISP.
At the end of the day if someone cannot handle a flame on a forum (I assume) and want to sneak around with email (ie. no-one else sees how childish they are) then they deserve to be highlighted.
On the other hand, it could be a mate having a laugh with you, and you just don't realise. You t**t.
...notice the winky smiley....
Cheers
I am 99% probably lying in bed right now... so don't blame me for crappy typing