Ipstenu (Mika Epstein) on "Add constants DISABLE_FEED for fine tuning a blog."

Put these lines in your function.php:

function mysite_kill_rss() {
		wp_die( __('No feeds available, please visit our <a href="'. get_bloginfo('url') .'">homepage</a>!') );
	}

	add_action('do_feed', 'mysite_kill_rss', 1);
	add_action('do_feed_rdf', 'mysite_kill_rss', 1);
	add_action('do_feed_rss', 'mysite_kill_rss', 1);
	add_action('do_feed_rss2', 'mysite_kill_rss', 1);
	add_action('do_feed_atom', 'mysite_kill_rss', 1);
	}

This stops all feeds.