I know what you want and I can help :)
Assuming you have not renamed wp-comments-post.php to something else. I have, so you never know...
RewriteCond %{HTTP_REFERER} "!^http://www.domain.org/.*$" [NC]
RewriteCond %{REQUEST_URI} ".*wp-comments-post.php$"
RewriteRule .* - [F]
Change domain.org to your domain name, and you should have what you want. And obviously, you might need to mess with the regex for your particular set-up, ie, blog.domain.org or not using the www. etc..
This is, by far, one of the smartest and simplest things people can do to ease the spam.
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that 3 line snip goes in your .htaccess
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johnoz,
Unfortunately, it is possible to send an http_post and using the proper variables, to insert comments from a remote site.
Its a VERY common spammer technique, and log watchers can easily see those attempts: theyre hits to wp-comments-post.php with a referer other than your site.