OK, something just isn't right with this forum in the last couple days or so - I've seen spam posts popping up, and now strange posts that don't fit the thread...
I remember before the switch to the SMF type of forum that this was a problem, and now it appears to be happening again. I'm not suggesting any switch to another forum system again, as I suspect that just about any type of forum might be vulnerable to some degree to this type of thing.
I'm afraid I really don't have a concrete solution to offer, but I will say that I've seen some forums fight this in different ways, some of them more successful than others. I don't claim to be an expert on the subject, but I've seen some forums where the first category is labeled "General" or "General discussion" and comes with a note to not post there, that anyone posting there gets deleted, thereby creating a spam trap.
I've also seen some forums where they customize some of the forum software code, or they do something extra to verify member accounts and insure they are not bots, although I really don't know how those work.
In the meantime, for what it's worth, if there's anything I can do on my end, I'll do what I can - so far in the past I've simply clicked on the "report to moderator" option.