A trick I've seen a lot when I was a community moderator, was forum posts that originally looked like valid discussions, quite often even related to what the forum was about, and that they edit the forum post 2 (or more) months later and remove the original discussion and replace it with a spam message. After all, they only wanted to fool search engine spider bots, and since months old forum posts are rarely seen by users, they are not very likely to be reported, and moderators won't easily see them either. Spiders are still likely to pick them up, though. This shows how clever spammers are.
And you're correct. There is no real way to stop them.