There is nothing defensible about this. Absolutely nothing. The post in which Whelan outs publius doesn't even contain a response to his arguments, just ad hominem attacks and claims that publius - who's a law professor - has "a dismal understanding of the legal matters he opined on" (which, judging from Whelan's hilarious misunderstanding of the role of policy in the legal system, seems like all too much projection). And his claim that using a psuedonym displays some sort of "cowardice" is all so much macho posturing, with no understanding of the damage non-anonymous blogging can do to one's legal or academic career (or, in publius' case, to relations with family).
Unlike publius, I'm no lawyer, so I can't really suggest a recourse other than explaining just how abhorrent Whelan's actions are - which he has done, in characteristically eloquent fashion. But it's truly horrifying to see someone who fashions himself an intellectual - and who leads an at least marginally respectable think tank - sink to such depths of harassment and character assassination. Whelan is not trying to enlighten his readers; he is trying to disrupt, if not destroy, publius' professional and personal life. As someone who, for four years, had to rely on others' willingness to respect my anonymity in order to experience some semblance of a normal, private adolescence, few things offend me more.
But I think it behooves Whelan's hosts - the editorial staff of National Review Online - to consider whether they want someone who personally harasses his interlocutors on their website, let alone their payroll. At the very least, I think they owe their readers an explanation if they choose to maintain their association such a bad blogospheric citizen.