bbp_trash_forum_topics( int $forum_id )

function bbp_trash_forum_topics( $forum_id = 0 ) {

	// Validate forum ID
	$forum_id = bbp_get_forum_id( $forum_id );
	if ( empty( $forum_id ) )
		return;

	// Allowed post statuses to pre-trash
	$post_stati = implode( ',', array(
		bbp_get_public_status_id(),
		bbp_get_closed_status_id(),
		bbp_get_pending_status_id()
	) );

	// Forum is being trashed, so its topics and replies are trashed too
	$topics = new WP_Query( array(
		'suppress_filters' => true,
		'post_type'        => bbp_get_topic_post_type(),
		'post_parent'      => $forum_id,
		'post_status'      => $post_stati,
		'posts_per_page'   => -1,
		'nopaging'         => true,
		'fields'           => 'id=>parent'
	) );

	// Loop through and trash child topics. Topic replies will get trashed by
	// the bbp_trash_topic() action.
	if ( !empty( $topics->posts ) ) {

		// Prevent debug notices
		$pre_trashed_topics = array();

		// Loop through topics, trash them, and add them to array
		foreach ( $topics->posts as $topic ) {
			wp_trash_post( $topic->ID, true );
			$pre_trashed_topics[] = $topic->ID;
		}

		// Set a post_meta entry of the topics that were trashed by this action.
		// This is so we can possibly untrash them, without untrashing topics
		// that were purposefully trashed before.
		update_post_meta( $forum_id, '_bbp_pre_trashed_topics', $pre_trashed_topics );

		// Reset the $post global
		wp_reset_postdata();
	}

	// Cleanup
	unset( $topics );
}