Update: Be sure to read the comments. There are lots of good suggestions on solving this problem. Here's what I did to stop spam storms

About three times per day my server gets hit my a comment storm. Someone with a botnet is trying to spam my blog and they're going about it stupidly. They don't get any comments through because of a simple textual CAPTCHA that I installed in June.

The storm occurs because the spammers try to post over 100 comments in the space of about 1 minute from five or six different IP addresses. Naturally, the load average on my server shoots up to unacceptable levels. I'm stymied about how to combat this. The IP addresses are different every time. The browse tag is MSIE, so you can't filter on that. If you have ideas, let me know.