delete an author's posts

mass-delete-author.php

<?php
function mass_delete_author() {

	$args = array(
		'fields'		=> 'ids',
		'author'		=> 31,
		'posts_per_page'	=> 100,
		'no_found_rows'		=> true,
		'cache_results'		=> false
	);

	$posts = get_posts($args);

	foreach ($posts as $post_id ):
		wp_delete_post( $post_id, true );
	endforeach;

}