Exclusion of categories not working

Try it with this:

<?php
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
query_posts('cat=-15&paged='.$paged);
?>

ah good thinking. Apply the page you are on as well as the exclusion.

Still odd though that the first bit of code does not work.

Maybe another loop is conflicting with the main loop. try it with this:

wp_reset_query();
global $query_string;
query_posts($query_string . '&cat=-15');

Or try as on : https://codex.www.ads-software.com/Function_Reference/query_posts

global $wp_query;
$args = array_merge( $wp_query->query, array( 'cat' => -15 ));
query_posts( $args );