How to exclude a category from the wordpress default home page

Go to the wordpress theme top level directory eg:

yourdomain.com/httpdocs/wp-content/themes/twentytwelve/index.php

Replace:

<?php if ( have_posts() ) : ?>

                     <?php /* Start the Loop */ ?>

With this:

<?php if ( have_posts() ) : ?>

                         <?php query_posts($query_string .'&cat=-31'); /*EDIT TO REMOVE PRIVATE CAT 31*/ ?>

			<?php /* Start the Loop */ ?>

Where Cat = [whatever category you want to exclude].