I applied a patch to my .Text installation as recommended by Scott Watermasysk in this entry of his blog.
New comments will now have the rel="nofollow" attribute applied, thus preventing Google (and others) from indexing the link and giving the comment spammers a higher page rank.
I found a slight problem with the patch. It works for URLs within the body of the comment but if the user specifies a URL in the URL field, it doesn't modify that URL. Thus you can still comment spam me, but only one URL at a time. I posted a comment in Scott's blog about this.
In any case, I doubt this will stop the comment spam anyways. It may well be good enough for the spammers to continue. Despite the fact that their Google page rankings won't increase as a result, by spamming enough sites, they'll get enough exposure on enough blogs (et all) that enough users will click through. It's the same way with email spam. All it takes is a very small percentage of suckers to bite.
This does take away one of the key motivators to comment spam. I will probably add a CAPTHA tool later after some investigation.