Block all WordPress comments that try include a url.

gistfile1.php

+// check honeypot field when a new comment is submitted
+// we hide the url field from normal visitors via css
+function at_pre_comment_check() {
+
+    if ( ! empty( $_POST['url'] ) )
+        wp_die( "Comment rejected because it triggered our spam filter. Disable any auto form filler and try again." );
+
+}
+add_action( 'pre_comment_on_post', 'at_pre_comment_check' );