bbp_delete_topic( $topic_id )

function bbp_delete_topic( $topic_id = 0 ) {

	// Validate topic ID
	$topic_id = bbp_get_topic_id( $topic_id );

	if ( empty( $topic_id ) || !bbp_is_topic( $topic_id ) )
		return false;

	do_action( 'bbp_delete_topic', $topic_id );

	// Topic is being permanently deleted, so its replies gotta go too
	// Note that we get all post statuses here
	$replies = new WP_Query( array(
		'suppress_filters' => true,
		'post_type'        => bbp_get_reply_post_type(),
		'post_status'      => array_keys( get_post_stati() ),
		'post_parent'      => $topic_id,
		'posts_per_page'   => -1,
		'nopaging'         => true,
		'fields'           => 'id=>parent'
	) );

	// Loop through and delete child replies
	if ( ! empty( $replies->posts ) ) {
		foreach ( $replies->posts as $reply ) {
			wp_delete_post( $reply->ID, true );
		}

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

	// Cleanup
	unset( $replies );
}