Exclude specific tag or tags from related posts?

Question

The Codex has almost exactly what you are asking:

$args = array(
    'post_type' => 'post',
    'tax_query' => array(
        'relation' => 'AND',
        array(
            'taxonomy' => 'movie_genre',
            'field' => 'slug',
            'terms' => array( 'action', 'comedy' )
        ),
        array(
            'taxonomy' => 'actor',
            'field' => 'id',
            'terms' => array( 103, 115, 206 ),
            'operator' => 'NOT IN'
        )
    )
);
$query = new WP_Query( $args );

You just need to remove the couple of parts you don’t need, and of course change the post_type and taxonomy details to fit your data.

$args = array(
    'post_type' => 'post',
    'tax_query' => array(
        array(
            'taxonomy' => 'post_tag',
            'field' => 'id',
            'terms' => array( 103, 115, 206 ), // change these
            'operator' => 'NOT IN'
        )
    )
);
$query = new WP_Query( $args );

Best I can do with the minimal detail provided in the question.

Based on new information in an edit to the question:

The 'tag' => $rel_tagnames, pattern you are using is deprecated. You should be using a tax_query as above. Since you are already including a list of tags to search, you shouldn’t need to exclude any. When you include specific tags, others are de facto excluded. So what you would want is this:

if ($rel_tags) {
    $relatedargs = array(
        'ignore_sticky_posts' => 1,
        'post__not_in' => array($id),
        'showposts' => $relatednumber,
        'orderby' => 'rand'
    );
    // Get list of tag names and set arguments for loop
    foreach($rel_tags as $rel_tag) {
      // exclude some tags
      $excluded_tags = array (1,2,3);
      if (in_array($rel_tag->term_id,$excluded_tags)) continue;
      $rel_tagnames[] = $rel_tag->term_id;
    }
    $relatedargs['tax_query'] = array(
      array(
        'taxonomy' => 'post_tag',
        'field' => 'id',
        'terms' => $rel_tagnames,
      )
    )
}

// the rest of your code

I changed that to use IDs instead of slugs. It is probably more efficient to search those than the slugs, though I haven’t benchmarked it, and converted to a tax_query. Tags are excluded when you build the “include” list, instead of trying to do that in the query itself.

Why does your function set globals instead of just returning data?

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