exclude posts with certain tag from related posts

Hi,

Open includes/related-posts.php and look for lines 44-51:

$my_query = get_posts(
                    array(
                        'tag__in' => $tag_ids,
                        'post_type' => get_post_type($this_id),
                        'showposts'=>$postcount, 'ignore_sticky_posts'=>1,
                        'orderby'=>'rand',
                        'post__not_in' => array($this_id))
                    );

Replace them by this:

$my_query = get_posts(
                    array(
                        'tag__in' => $tag_ids,
                        'post_type' => get_post_type($this_id),
                        'showposts'=>$postcount, 'ignore_sticky_posts'=>1,
                        'orderby'=>'rand',
                        'post__not_in' => array($this_id),
                        'tax_query' => array(
                            array(
                                'taxonomy' => 'post_tag',
                                'field' => 'slug',
                                'terms' => array('_YOUR_UNDESIRED_TAG_'),
                                'operator' => 'NOT IN'
                            )
                        )
                    ));

Change _YOUR_UNDESIRED_TAG_ by the tag you want to exclude.

Regards,
Josue