Exclude posts containing a particular tag

Hi
I am fairly new to WP and PHP so please forgive my lack of knowledge; Bascailly I’ve been setting up template pages to show posts which have been tagged with certain tags;

It looks like I’ve got working; however what I need to do now is the opposite; to exclude posts containing certain tags; I have paasted the current code which includes certain tagged posts here, can anyone suggest how to modify it to exclude them;
Thanks for your help
Darren

<?php
/*
Template Name: Helens Posts
*/
get_header(); ?>

<div id="content_box">
	<div id="content" class="posts">
	<?php
$term = get_term_by('slug','helen', 'post_tag');
$args=array(
  'tag__in' => array($term->term_id),
  'post_type' => 'post',
  'post_status' => 'publish',
  'posts_per_page' => -1,
  'caller_get_posts'=> 1
);
$my_query = null;
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) {

while ($my_query->have_posts()) : $my_query->the_post(); ?>
<h2><?php the_title(); ?></h2>
<h4><?php the_time('F jS, Y') ?><!-- by <?php the_author() ?> --> &middot; <a>#comments"><?php comments_number('No Comments', '1 Comment', '% Comments'); ?></a> &middot; <?php if (!is_attachment()) { the_category(', '); } ?> <?php edit_post_link('Edit', ' &middot;', ''); ?></h4>
<div class="entry">
					<?php the_content('<p>Read the rest of this entry &raquo;</p>'); ?>
					<?php link_pages('<p><strong>Pages:</strong> ', '</p>', 'number'); ?>
				</div><?php if (!is_attachment()) { ?><p class="tagged"><strong>Tags:</strong> <?php the_tags('','&middot;',''); ?></p><?php } ?>
<div class="clear"></div>
<?php
  endwhile;
}
wp_reset_query();  // Restore global post data stomped by the_post().
?>	

 </div>
<?php get_sidebar(); ?>
</div>

<?php get_footer(); ?>