WordPress Contact Form 7 Spam Filter

wpcf7-spam-filter.php

<?php

/**
 * Contact Form 7 Spam Filter
 */
add_filter( 'wpcf7_spam', function( $spam ) {
    if ( $spam )
        return $spam;

    $spam_key_words = [
        'demo'
    ];

    $found_spam_key_word = function ( $string, $spam_key_words ) {
        foreach ( $spam_key_words as $spam_key_word )
            if ( stripos( $string, $spam_key_word ) !== false )
                return true;

        return false;
    };

    return (
        $found_spam_key_word( $_POST['subject'], $spam_key_words )
        ||
        $found_spam_key_word( $_POST['message'], $spam_key_words )
    );
}, 10, 1 );