Already some months ago Mark West tested the Akismet spam protection suite. It was included into the CVS-version of EZComments and worked quite nicely. But the spam detector is a feature that many modules need. So it has now become a module of its own. It's not yet ready, but I hope it will be soon available for inclusion into guestbooks, comments, forums aso.
A module that Mark West released yesterday is "Bad Behaviour". "Bad Behavior is a PHP-based solution for blocking link spam and the robots which deliver it.
Bad Behavior complements other link spam solutions by acting as a gatekeeper, preventing spammers from ever delivering their junk, and in many cases, from ever reading your site in the first place. This keeps your site’s load down, makes your site logs cleaner, and can help prevent denial of service conditions caused by spammers." -- Bad Behaviour Homepage
As you include it into the pnAPI.php at earliest possible position, it stops Postnuke loading its higher functions when it detects a spam bot access. This means that all detected access cause only little traffic.
IMHO a combination of Akismet and Bad Behaviour will reduce the amout of spam on our sites without building barriers also for normal users. But we will have to live with spam, if we want to keep up our sites.