Several years ago Google introduced the rel="noFollow" to be placed in <a> tags. When spiders see the link they don't follow it, which cuts off most of the reasoning behind comment spam, to boost ratings.
It slowed, but did not stop comment spam.
I have an idea. A new tag that can be applied to the div or td element that wraps the post - rel="spam". The forum software would have the ability that when a post is flagged as spam it gets a CSS class on its div to hide it from users, and the rel tag for the spider to understand that the contents of the div are spam...
Now the spider can take the contents and see how many other forums have the same content with the same (or similar) marking. Leave it up to Google and Microsoft to determine the exact implementation - the idea here is clear. The search engine can penalize the sites who's links appear in the post or remove them from rankings entirely.
Since comment spam is, at heart, a rankings game, this might ice their heels a bit.
To work it would require the major forum and blog software package makers to implement the toggle, and the search engines to then read the spam and look for matches. A post being marked as spam on one site obviously shouldn't penalize anyone, but if 90 different sites in 3 days get the same post and that post gets marked spam then odds are you have a spammer.