function myextensionTinyMCE($init) {
    // Command separated string of extended elements
    $ext = 'span[id|name|class|style]';

    // Add to extended_valid_elements if it alreay exists
    if ( isset( $init['extended_valid_elements'] ) ) {
        $init['extended_valid_elements'] .= ',' . $ext;
    } else {
        $init['extended_valid_elements'] = $ext;
    }

    // Super important: return $init!
    return $init;
}

add_filter('tiny_mce_before_init', 'myextensionTinyMCE' );

Quelle: wordpress.org/support/topic/prevent-span-tags-from…