akseli_a

You have to add a few lines of code in ‘The Loop’ that runs as the index page loads. The file in question that must be modified is: wp-content/themes/spun/index.php

ORIGINAL:

get_header(); ?>

		<div id="primary" class="content-area">
			<div id="content" class="site-content" role="main">
			<?php if ( is_home() && ! is_paged() ) : ?>
				<h2 class="site-description"><?php bloginfo( 'description' ); ?></h2>
			<?php endif; ?>

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

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

				<?php while ( have_posts() ) : the_post(); ?>

and change that to:
MODIFIED:

<div id="primary" class="content-area">
			<div id="content" class="site-content" role="main">
			<?php if ( is_home() && ! is_paged() ) : ?>
				<h2 class="site-description"><?php bloginfo( 'description' ); ?></h2>
			<strong><?php endif; ?>
			<?php if ( is_home() ) {
			query_posts($query_string . '&cat=-3');</strong>
			}
			?>
			<?php if ( have_posts() ) : ?>

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

				<?php while ( have_posts() ) : the_post(); ?>

What you want to modify is: ‘&cat=-3’); and add -3,-5,-8… etc

More detailed instructions: http://codex.ww.wp.xz.cn/The_Loop