Remove url field from the comment form and automatically spam comments with urls

functions.php

<?php
function em_remove_comment_fields($fields) {
    unset($fields['url']);
    return $fields;
}
add_filter('comment_form_default_fields','em_remove_comment_fields');

function em_url_spamcheck( $approved , $commentdata ) {
  return ( strlen( $commentdata['comment_author_url'] ) > 1 ) ? 'spam' : $approved;
}
add_filter( 'pre_comment_approved', 'em_url_spamcheck', 99, 2 );
?>