bbp_trash_topic( $topic_id )

function bbp_trash_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_trash_topic', $topic_id );

	// Topic is being trashed, so its replies are trashed too
	$replies = new WP_Query( array(
		'suppress_filters' => true,
		'post_type'        => bbp_get_reply_post_type(),
		'post_status'      => bbp_get_public_status_id(),
		'post_parent'      => $topic_id,
		'posts_per_page'   => -1,
		'nopaging'         => true,
		'fields'           => 'id=>parent'
	) );

	if ( !empty( $replies->posts ) ) {

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

		// Loop through replies, trash them, and add them to array
		foreach ( $replies->posts as $reply ) {
			wp_trash_post( $reply->ID );
			$pre_trashed_replies[] = $reply->ID;
		}

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

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

	// Cleanup
	unset( $replies );
}