If an href tag includes the rel="nofollow" attribute, well-behaved search engines won’t follow the links they represent when spidering. So if there was a way to automatically modify the links that comment spammers leave in comments, their chief goal — raising their standings in the search engines — would be deflated.
SixApart has just released the nofolllow plugin, which scans incoming comments and adds rel="nofollow" to each embedded link automatically. Normal users are not affected — they can still click the links. But the simple presence of links to spammer’s sites will do nothing whatsoever for their GoogleRanks.
The downside, as I see it, is that for this to be effective, it must be intalled in the majority of weblogs. Spammers need to understand that their campaigns are flaccid, and that won’t be true until most of the world is using a solution like this.
Just installed nofollow at birdhouse and at the J-School.