Here are a couple ways. I'll keep updating...

    add_action( 'template_redirect', function() {
    
    	if ( in_array( true,
    		array (
    			is_feed(),
    			is_trackback(),
    			is_embed(),
    		) ) ) {
    		wp_die( __( "NO SOUP FOR YOU!" ) );
    	}
    } );
    
    add_action( 'init', function() {
    
    	$feeds = array (
    		'do_feed',
    		'do_feed_rdf',
    		'do_feed_rss',
    		'do_feed_rss2',
    		'do_feed_atom',
    	);
    
    	foreach ( $feeds as $feed ) {
    		remove_action( $feed, $feed );
    	}
    } );