The following hack will exclude a specific category from the searching options. For example, you don’t want visitor’s to be able to search through category X, you can exclude the category. Open up the search.php file, and add the following code before the loop starts.
<?php if( is_search() ) : $paged = (get_query_var('paged')) ? get_query_var('paged') : 1; query_posts("s=$s&paged=$paged&cat=-3"); endif; ?>
In order to exclude category X, find it’s ID number, and paste it where -3,-8 is. By putting the – sign before the numbers, you are excluding them from the loop.