If you are using functions.php script to register custom post type, you should declare false to 'has_archive' => true,.
function custom_post_type() {
$labels = array( ... );
$args = array(
// you have to set it to False.
'has_archive' => false,
);
register_post_type( 'post_type', $args );
}
add_action( 'init', 'custom_post_type', 0 );