bbp_delete_forum_topics( int $forum_id )

function bbp_delete_forum_topics( $forum_id = 0 ) {

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

	// Forum is being permanently deleted, so its content has go too
	// Note that we get all post statuses here
	$topics = new WP_Query( array(
		'suppress_filters' => true,
		'post_type'        => bbp_get_topic_post_type(),
		'post_parent'      => $forum_id,
		'post_status'      => array_keys( get_post_stati() ),
		'posts_per_page'   => -1,
		'nopaging'         => true,
		'fields'           => 'id=>parent'
	) );

	// Loop through and delete child topics. Topic replies will get deleted by
	// the bbp_delete_topic() action.
	if ( !empty( $topics->posts ) ) {
		foreach ( $topics->posts as $topic ) {
			wp_delete_post( $topic->ID, true );
		}

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

	// Cleanup
	unset( $topics );
}