In single.php simply get rid of the call to the sidebar >>>
<?php get_sidebar(); ?>
On individual post pages the sidebar will not appear. Of course it may look a little strange now so you might want to take it a few steps further. For instance, creating 2 new classes: 1 for the index page and 1 for the post pages. Example, index.php you could have something like,
<div id="content" class="narrowcolumn">
which is defined in the style sheet as being 400px wide while in single.php you could have,
<div id="content" class="widecolumn">
which is defined in the style sheet as being 500px wide. Study the default Kubrick theme to see how this works.