Simply paste the following lines into your .htaccess file. This file is located at the root of your WordPress install.
Remember to always make a backup of your .htaccess file before editing it so, you’ll be able to restore it if something went wrong.
Don’t forget to replace yourdomainname on line 5 by your real domain name.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} .wp-comments-post\.php*
RewriteCond %{HTTP_REFERER} !.*yourdomainname.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule (.*) ^http://%{REMOTE_ADDR}/$ [R=301,L]
</IfModule>
Once you saved your .htaccess file, spam bots will not be able to access your wp-comments-post.php file directly. This will significantly reduce the amount of spam received on your blog.
Thanks to AllGuru.net for the tip!
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Reduce spam on your WordPress blog by using .htaccess