to exclude a certain category of posts from the WP loop:

<?php query_posts(“cat=-46”); ?>
<?php if (have_posts()) : ?>
blah blah blah. and then close with the old:
<?php
//Reset Query
wp_reset_query();
?>

To exclude certain categories from an RSS feed on another webpage (like a non-wp homepage):

http://yourdomain.com/blog/feed/?cat=-46