For this specific spammer, ban the words 'admin' 'root' etc, case-insensitive, in the username. This doesn't solve all of it (including the most recent spam..)
Disable sending of Emails on PM to prevent the entire userbase from being alerted. We need stronger anti-spam countermeasures. (ReCaptcha, hidden registration fields, StopForumSpam plugins, maybe even user moderation..)
I'm surprised a IPB1.3 forum has survived this long, but my SMF is getting hit anyway.. A forum upgrade won't solve the problem completely.