Seeking a way to kill "Zombie" emails that cannot be deleted

For the second time in three months, I've gotten a spam email that cannot be deleted. It has been dumped in the Spam folder by SpamSieve, but once there, it cannot be removed. The mail account is a POP acct, and mail is downloaded from the server so in theory should not be reappearing from there.

The email address is bogus, from no one I know. The email message contains no Content, but indicates there is an attachment via the paperclip icon. Open it, however, and there is nothing there (though I knew it was zombie mail when it arrived, as it's the second time this has happened—the first vanished on its own after about a month, so it appears to have a preset termination date; what it's doing in the interim, I don't know).

I have attempted trashing it every way I know how. The response is always that said piece of mail cannot be erased because its attachment does not exist. I even manually renamed a .emix file in Mail to correspond to this. No soap.

And now have gone into the User library, trashed the plist and folder that seem to be connected to this. And the file seemed to vanish, so I quit Mail. The next time I opened mail the bogus email returned. And as the screen shot shows, one became two, though there is only one showing.

I have rebuilt the mailboxes. The only thing I haven't done is try to kill the thing via Terminal (if such a thing is even possible).

Given similar reports seem to be turning up here (Cannot move or delete defective email messages.) for example, I have to conclude that this is some sort of hideously annoying invasive file that's getting through firewalls and spam filters, and for the life of me at this point, if I could find the sender, I would bloody shoot the ******* myself.

Any advice would be appreciated.

-gf