I finally caught up to the spammer to the point where I was marking them as fast as they were making them, and I noticed they were jumping around from thread to thread, only making a single post before jumping to another. While the forums I have marked are listed alphabetically, however, the spammer jumps from Fallout to Neverwinter Nights before going to Might and Magic, which happens to be the way they're listed when you're signed out.
So for anyone who wants to follow along and mark threads as the spambot makes them, the order is Witcher 2, Baldur's Gate, Fallout, Neverwinter NIghts, and Might and Magic.
It might be worth making all game subforums hidden behind the "show all 652 forums" button to keep the spammers from hitting areas where fewer people are around to flag spam in the future.