Exclude posts with certain tag from loop

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<?php

// Query_Posts in your loop
query_posts(
 array(
  'tag__not_in'	=> array(get_tag_id_by_name('teaser')),
);
 
// The following function goes into your functions.php
function get_tag_id_by_name($tag_name) {
	global $wpdb;
	return $wpdb->get_var("SELECT term_id FROM ".$wpdb->terms." WHERE `name` =  '".$tag_name."'");
}
?>

Instructions:

This snippet allows you to exclude posts tagged with a certain tag from the loop. It does not require the tag-id, but accepts the tag-name and resolves the name to the id that belongs to the tag for you.