Joi Ito has some interesting thoughts on classifying comment spam. Knowing where to draw the line, as he puts it, is the first step in maintaining an equitable deletion policy. The resulting discussion is worth a read.
Of particular interest: Adam Lasnik's initial response to Joi's post strengthens the case against URL proxies by emphasizing what we lose:
Many bloggers' admirable sites have become popular from the GoogleJuice (PR) they've attained from their posted comments. And this, IMHO, is a good thing -- everyone has ended up a winner. These thoughtful bloggers have contributed interesting and useful commentary to blog entries, and in return, they've enriched those blogs... and also boosted the popularity of their own blogs.
[R]emoving the opportunities for the 'legitimate' folks to enjoy promotional side benefits from their sincere contributions is more unpleasant, I believe, that the original spam issues...