rel="nofollow" supported by Google
According to this post at Google Blog, Google supports rel="nofollow" in links. What this means is that if you have a link like this
<a href="http://somedomain/somepage" ref="nofollow">
in your pages, Google will not follow this link. Adding by default ref="nofollow" to urls included in blog comments, refferers, etc should make blog-smamming useless since what these spammers want is to increase their PageRank (or search engine valuation in general).