Automatically spam comments with a very long url

Open your functions.php file and paste the code below in it. This code will automatically mark as spam all comments with an url longer than 50 chars. This can be changed on line 4.

<?php

  function rkv_url_spamcheck( $approved , $commentdata ) {
    return ( strlen( $commentdata['comment_author_url'] ) > 50 ) ? 'spam' : $approved;
  }

  add_filter( 'pre_comment_approved', 'rkv_url_spamcheck', 99, 2 );

?>

Thanks to CSS Tricks for the code snippet!