Coming from someone who's a moderator on another discussion forum online, I definitely understand that there will be spammers. But normally, the ones I've seen before just make new posts...
These seem to be all sorts of strange, pulling up dead threads and posting in them...
Sorry guys, we're pulling them as fast as we can.
A reminder that you can help us out by reporting all spam posts that you see by clicking on the at the top of the post.
As for spamming old threads, all I can figure its the same guy doing most of it and he probably has them saved in favorite places.
I doubt there's any person-interaction with any of the spammers. It's all scripts that automatically sign up and post in random threads. Or maybe they use hardcoded thread ids, which explains the old threads..
If there is an option in the admin-portion to use email notification upon signup, or even use that randomized graphic-code (you know the kind (type the code to the left to continue) to verify it's an actual person, it may help to prevent these spammers..