wpian-help-terms-only.php

<?php
/**
 * Show posts which are only under uncategorized term
 * 1 is the id of Uncategorized term
 */
function op_show_only_uncategorized( $query ) {
	if ( is_admin() || ! $query->is_main_query() || ! $query->is_category( 1 ) ) {
		return;
	}
  
  // Query all the category terms excluding Uncategorized term
	$categories = get_categories( array(
		'exclude' => '1',
		'fields'  => 'id=>slug'
		) );
  
	$query->set( 'category__not_in', array_keys( $categories ) );
}
add_action( 'pre_get_posts', 'op_show_only_uncategorized' );