Warning! The Cookies for Comments plugin hasn’t been updated in a long time and currently doesn’t work on sites running anything newer than PHP 7.2. You’re better off using Akismet.

This plugin will add a randomly generated stylesheet url to your blog’s theme. When a browser requests that stylesheet a cookie is set. If that person then tries to comment, the cookie must be set.

WordPress MU sites can use the plugin to protect themselves from spam blogs by editing the .htaccess and adding the following code where XXXXXX is replaced by the cookie value. See the plugin readme.txt for more info.

RewriteCond %{HTTP_COOKIE} !^.*XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.*$
RewriteRule ^wp-signup.php - [F,L]

Download: cookies-for-comments.zip
Support: Cookies for Comments Support Forum