Comment Spam Storms

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.