John Battelle is getting increasingly frustrated in his battle with weblog spam. It is a huge problem. But, as Mark Pilgrim points out, the common solution to weblog spam, blacklists and filters, isn't likely to work.
Instead, the focus should be on slightly increasing the cost of posting. For example, some sites put up an image of a random code that needs to be typed in to register or post a comment. The image is difficult for an automated process to read, so most spammers won't bother trying. Other sites require confirmation that you own an e-mail address by sending you an e-mail with a link that needs to be clicked for your registration or post to be accepted. Both of these methods incrase the cost to spammers with only a minor annoyance to non-spammers.
Blog comment spam exists because it works. Increasing the costs, attacking the economics of spam, is the best way to solve the problem.