Very true.

The best you can do is allow the post to go thru and make it appear to the spammer that they have succeeded. Let them view their post on the homepage and go away with a nice warm feeling of satisfaction, secure in the knowledge that they don't need to improve their algorithm at all for this site.

It's pretty straightforward to modify your display mechanism to include spam posts if and only if they originate from the same IP address as the viewing client.

I implemented this on Blogabond, and it had two positive effects: My Spam corpus is growing at a faster rate (thus making it more effective), and the sophistication level of the average attack has dropped sharply.