Now, you say you weren't on the board when this happened, so for all you know, he posted at normal speed during the day and none of your mods were around to do anything about it. I had an email addressed to me at support and the sender said that the posts had been there for ages and nobody was attending to the problem.
I would be very surprised indeed if our security had been breached by some "script kiddie".
I take it that you have guest posting enabled on your board? If so, you really need to disable this completely, in view of the trouble you've been having. That is the very least you can do.
Actually, if you don't want full MBA (and maybe you should
have it for a while), then at least restrict all posting to members of the board, except for one forum, where newcomers may post to introduce themselves. We give admins the tools to defend themselves, to cater for just this sort of problem.