bbp_remove_user_topic_subscription( int $user_id, int $topic_id )

function bbp_remove_user_topic_subscription( $user_id, $topic_id ) {
	if ( empty( $user_id ) || empty( $topic_id ) ) {
		return false;
	}

	$subscriptions = (array) bbp_get_user_subscribed_topic_ids( $user_id );
	if ( empty( $subscriptions ) ) {
		return false;
	}

	$pos = array_search( $topic_id, $subscriptions );
	if ( false === $pos ) {
		return false;
	}

	array_splice( $subscriptions, $pos, 1 );
	$subscriptions = array_filter( $subscriptions );

	if ( !empty( $subscriptions ) ) {
		$subscriptions = implode( ',', wp_parse_id_list( $subscriptions ) );
		update_user_option( $user_id, '_bbp_subscriptions', $subscriptions );
	} else {
		delete_user_option( $user_id, '_bbp_subscriptions' );
	}

	wp_cache_delete( 'bbp_get_topic_subscribers_' . $topic_id, 'bbpress_users' );

	do_action( 'bbp_remove_user_topic_subscription', $user_id, $topic_id );

	return true;
}