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Many spambots simply hit the wp-comments-post.php and wp-trackback.php files directly, without scanning your site to find the real names of these scripts. You can foil such bots by renaming these files.
This trick won't catch all incoming spam. However, the spam that it does catch will be disposed of without chewing up your server resources, and this hack is totally transparent to your readers.
Let's say you want to rename your file wp-comments-die-spam-die.php.
These changes may be overwritten if you install a new version of your theme. Changes made to the WordPress Default Theme may also be overwritten when you upgrade WordPress.
If you expect to change the name of your comments script on a regular basis, you can make it easier by using PHP to call up the new name from a single, central location.
Just create a text file called something like comment-script-name.txt, and place it in your main WordPress directory. The file should contain just the new name of your wp-comments-post.php script, e.g., wp-comments-die-spam-die.php with no spaces or line breaks. Then, replace references to wp-comments-post.php with <?php include('comment-script-name.txt'); ?>.
Then if spammers start directly attacking wp-comments-die-spam-die.php, you need only rename the file and change the name in comment-script-name.txt.
Let's say you want to rename your file wp-trackback-example.php.
If you upgrade WordPress, these changes will be overwritten and you will need to redo them.