You always can use pre_get_postspre_get_posts
or posts_where hooks (you can find documentation on Codex)posts_where
hooks.
Here you have an example of using pre_get_postspre_get_posts
to exclude some posts from search results: http://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts#Exclude_Pages_from_Search_Results
It's better solution, because you won't make another, redundant SQL query.