Quick&simple anti-spam for WordPress

os-spam.php

<?php
/*
Plugin Name: OS Spam Comments Filter
Description: antispam
Author: dimas@odminstudios.ru
Version: 1.1
*/


function verify_comment_meta_data( $commentdata ) {
    if ( !is_user_logged_in() && (!isset( $_POST['city'] ) || $_POST['city']!='Москва') ){
        wp_die( 'Вы не прошли проверку на спам.' );
    }
    return $commentdata;
}
add_filter( 'preprocess_comment', 'verify_comment_meta_data' );

function verify_comment_meta_data_field(){
	if( !is_user_logged_in() ) {
			?><p class="check_field">
			<label for="city">Антиспам: столица нашей страны? (Москва)
			<input class="city" name="city" type="text" value="" />
			</label>
		</p>
<script type="text/javascript">
jQuery(document).ready(function(){
	jQuery('.check_field input').val('Москва');
	jQuery('.check_field').hide();
});
</script>
		<?php
	}
}
add_action( 'comment_form', 'verify_comment_meta_data_field' );