One of the things that the spammers have in common is that they join just to spam, so their post count is usually 1 in their first spam message. Would there be a way to ensure that new members are limited to a certain number of posts a day until their post count reaches some arbitrary number?
The spammer who tries to post to multiple old threads, mucking up the "All Topics" list wouldn't be able to. If an automated system then moved all their posts to a "Spam" thread, it could then be perused by mods incase someone innocent had been caught by the system.
I have no idea if this is possible, but I'm sure I've seen new member restrictions on other internet fora.