function SearchFilter($query)
{
	if ($query->is_search)
	{
		$query->set('cat',-get_cat_ID('CategoryName'));
	}
	return $query;
}
add_filter('pre_get_posts','SearchFilter');

Found this bit of code all over the place, except it missed out a very crucial part of the code which was to add a "-" in front of the ID you wish to exclude.  Add this to functions.php in your theme folder.

Exclude specific category from Search Results in Wordpress

wordpress

PHP