I am actively working on the issue now, have upgraded the forum software and will look for some more spam prevention controls. Might mean sending all posts from new users into a moderation queue for a while.
@Chris Kelleher Is it possible to add the Spam cleanup option on the approval queue page? I can delete the posts easily enough in one go, but I can't send them to the spam cleanup without visiting each thread individually.
Reporting the posts is helpful when it's inside a thread rather than a thread of its own. But I go through every new post anyway so that's not so much of an issue.
Moderating the spam is tedious but not too hard, and if Chris sets all new users' posts to go to the moderation queue, along with the option to bulk spam report, then it should actually get easier from a moderation standpoint.