Paste the code below into your functions.php file. RSS feeds will not be available anymore after you saved the file.
unction digwp_disable_feed() {
wp_die(__('<h1>Feed not available, please visit our <a href="'.get_bloginfo('url').'">Home Page</a>!</h1>'));
}
add_action('do_feed', 'digwp_disable_feed', 1);
add_action('do_feed_rdf', 'digwp_disable_feed', 1);
add_action('do_feed_rss', 'digwp_disable_feed', 1);
add_action('do_feed_rss2', 'digwp_disable_feed', 1);
add_action('do_feed_atom', 'digwp_disable_feed', 1);
Thanks to Jeff Starr for the cool tip!