hkonno

I don’t know if you managed to find a solution to this. Also I’m not sure what you mean by Older Posts widget, but if you meant the default Archives widget, adding a filter to getarchives_where should work.

function my_arc_widget_exclude_category($x) {
	global $wpdb;
	$excl_cat = get_cat_id('category_to_be_excluded');
    $where_cond = !empty($x) ? $x . ' AND ' : 'WHERE ';
    $where_cond = $where_cond
        . "$wpdb->posts.ID NOT IN (SELECT p.ID FROM $wpdb->posts AS p "
        .  " INNER JOIN $wpdb->term_relationships AS r ON (p.ID = r.object_id)"
        .  " INNER JOIN $wpdb->term_taxonomy AS t ON (r.term_taxonomy_id = t.term_taxonomy_id)"
        .  " WHERE t.taxonomy = 'category' AND t.term_id IN ($excl_cat))";
    return $where_cond;
}
add_filter( 'getarchives_where', 'my_arc_widget_exclude_category' );

Replace category_to_be_excluded with the category you want to exclude and place the above code in your function.php. You might experience performance issue if you have many posts.