I looked up some of the addresses of visitors to deekayen.net who've attempted to leave spam, whether it be referrer, comment, or contact form spam. About half of them were listed in the SPEWS.org Level 1 list. Naturally, I decided I needed to block those operations from messing with any of my sites. From SPEWS:

The majority of the Level 1 list is made up of netblocks owned by the spammers or spam support operations themselves, with few or no other legitimate customers detected.

Two days later, I committed support for the Drupal troll module to import the SPEWS Level 1 & 2 lists, and the okean.com lists of netblocks owned by China and Korea. What you can do to blacklisted visitors:

• add a random stutter of 1-5 seconds to the page output
• drop form POST submission data
• always send blank pages
• always send a 404 not found message
• always redirect to an alternate URL

Of course, there's a whitelist to make sure you don't lock yourself out of your own site and you can lookup IP addresses in a search form to see if they match IP blocks in the blacklist. I don't have PostgreSQL to test support for that, but otherwise, it just needs some peer review to get branched for DRUPAL-4-7 in CVS.