I don't understand why I need to do this, but I'm enabling captchas on my comments. The majority of comments I get are in Chinese, and have about 20 periods at the end with links to .info websites. Reporting the posters as spammers hasn't stopped the comments, so hopefully this will.
Update: Fail! two spam comments have already appeared on this post. If Blogger doesn't fix this problem, I will move to WordPress.
Update2, Sept 7, 2010: Blogger has enabled automatic spam comment detection, and a comment spam box. I've not seen spam comments in the past week, and so far no spam comments in my inbox. I'm becoming optimistic. Foolhardy, I know.