Hi @Gualtiero3
Its very unusual for people to leave a place that quickly. Something had to be wrong. What happened here?
It was nice of you to offer a full refund. Maybe to nice to ne honest. Normally you would offer a partial refund but if the guest's complaint was really serious, then I would offer a full refund but it would have to be very very bad and grave.
You can't undo anything of these things - only take his feedback and do better in the future. I presume he wasn't unfair to you since you gave him the full refund.
Reviews cant be undone or written retroactively and once you offered a refund and he accepted the change of booking, it can't be undone either. You could ask him to pay for half or a whole day but I'll be honest and tell you that it won't happen. At least I wouldn't myself pay for half of a day if we previously agreed to a total refund.