Hide Categorie from main listing

I wish to hide one of my categories from the main news listing page but would still like to be able to show the news item when someone goes direct to a post from that categorie.

i assume i need to change the <?php if ($posts) : foreach ($posts as $post) : start_wp(); ?> in a way to denie the cat_id?

ive no idea how to do this in WordPress can anyone point me in the right direction?