@Marina481
In terms of the star rating, I would put this under communication, e.g. if a guest says they were booking for two and turned up with four without informing the host, I'd consider that very poor communication and also a breach of house rules (my rules do contain stuff that cover this).
In terms of preventing guests from doing this, well the system is set up to make it as quick and easy for them as possible with a minimal amount of stages/prompts and I believe this is very intentional. Airbnb wants them to book asap and not to have to think too much about it.
What is annoying is that a guest can search without entering the number of guests. The system will then automatically show them the price for one guest. Or at least that's how it appears to me. Because I have never used instant book when reserving an Airbnb, I don't know if there's some stage where the guest does have to specify the number of guests or not.
However, I feel like it's very easy for a guest not to enter the correct number of people, either intentionally or unintentionally. I used to have a maximum of two people per room, but with a higher price for two. In the search results, the listing shows as being for two people, but the price the guest will see until they change the search to two guests, is the price for one.
So, I would sometimes get guests booking for one and then referring to 'we' in their message. When I told them they had only booked for one and needed to change it and pay the additional charge, most were okay with this, but some got annoyed. Of course it's not the host's fault that Airbnb displays the listings like this, but I can understand why a guest could get confused and then feel like they are being ripped off.
Also, when I still had IB turned on, in theory guests were supposed to answer my pre-booking questions, the first of which was "Who are you travelling with and what is the purpose of your trip?". In reality, it's not a 'pre-booking' message as they can book instantly without any contact with the host and I found that the majority of guests did not answer some or all of the questions. So, it's misleading to hosts to tell them guests must answer your pre-booking questions to IB with you. They are not technically required to do any such thing!