I found a little trick that you can use. I copied this from dvorak.org/blog

Here is the short code running on the ctyme server for dvorak.org using WordPress-based blogging software. Altering it for other blog software and other blogs should be simple for anyone running Apache.

<location /blog/wp-comments-newpost.php>
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^.*dvorak.org/.*
RewriteRule ^.* http://www.ctyme.com/comment-spam.html
</location>


Essentially it makes the basic condition for any post rigid: it has to be coming from a link within the blog itself, the “comment” link. Most spam does not.

John claims that the spam count on his blog has dropped from 50-100 to 2 per day without any other tricks.

I will be implementing this on my blog soon. Not that I get any visitors, or spam, but worth a shot anyhow. :)