Exclude Category from Homepage

Exclude Categories From Your Home Page

Placing this code in your index.php file will cause your home page to display posts from all categories except category ID 3.

<?php
if (is_home()) {
query_posts(“cat=-3”);
}
?>

You can also add some more categories to the exclude-list(tested with WP 2.1.2):

<?php
if (is_home()) {
query_posts(“cat=-1,-2,-3”);
}
?>

from: http://codex.wordpress.org/Template_Tags/query_posts