Function to check if the listings is going to be shared is an spam message.

akismet_send_friend.php

<?php

function send_friend_post_akismet($item) {
    // we don't performe this action if akismet is not 
    if( !osc_akismet_key() ) {
        return false;
    }

    require_once osc_lib_path() . 'Akismet.class.php';
    $akismet = new Akismet(osc_base_url(), osc_akismet_key());

    $akismet->setCommentAuthor(Params::getParam('yourName'));
    $akismet->setCommentAuthorEmail(Params::getParam('yourEmail'));
    $akismet->setCommentContent(Params::getParam('message'));
    $akismet->setPermalink(osc_item_url());

    // Is Akismet saying it is spam?
    if( $akismet->isCommentSpam() ) {
        // define the message you want to show
        osc_add_flash_error_message('Spam message');
        header('Location: ' . osc_item_send_friend_url()); exit;
    }
}
osc_add_hook('pre_item_send_friend_post', 'send_friend_post_akismet');