Use the below code
<?php query_posts('cat=-3'); ?>
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<h3></h3>
<p><?php the_time('F jS, Y') ?></p>
<?php the_content(); ?>
<?php endwhile; ?>
In this code remove the category id with 3
If you need to remove more than one category use comma like below
-3, -4, -5
query_posts('cat=-3,-4,-5');