Exclude category/Show one in blog loop

// EXCLUDE
query_posts("cat=-6",  array( 'post_type' => 'post', 'paged' => $paged ) );

// or (in Columns theme):

$wp_query->query("cat=-15",'showposts=' . $limit . '&paged=' . $paged);

// SHOW ONE
query_posts("cat=2",array( 'post_type' => 'post','paged' => $paged ) );

// or (in Columns theme):

$wp_query->query("cat=15",'showposts=' . $limit . '&paged=' . $paged);