I think it's some script kiddie fucking around with a new toy. A Google search shows the same spam on multiple small forums, but it's not widespread, it's not hitting big sites, and it's not constrained to just one forum software. It's all very recent (nothing before a couple of weeks ago), too. The sites I'm looking at show the same slow, lackadaisical approach to the attack that we're seeing, nothing of the scale that a real attack would entail (hundreds or thousands of accounts all posting simultaneously). Someone with a small botnet or a single bot, testing it, finding out what it can do. That it goes dark for several hours between attacks suggests that it's either rotating targets, hitting a site for a while, then moving on to the next one in the list, or the user hasn't figured out how to fully automate it yet and has to be in front of the computer to make accounts and start the bot working. My gut tells me it's the latter.