@Lynn16
What I usually do in that kind of situation is I approve and then send a message to the guest along the lines of
"I am accepting on the basis that you CLEARLY understand that this reservation is for 1 guest, and you agree to fully respect my house rule about (1) no unregistered guest and you understand that (2) my listing is a private room in a shared home.
Before you proceed with your reservation, please reply that you have read all the detailed descriptions abour my home and you agree to abide by my house rules. - Once you agree to my terms, I will be happy to be your host 🙂 "
Usually, most questionable guests decide to stay somewhere else (big surprise~). If the guest does not confirm and the reservation happens, then I can contact Airbnb and ask them to cancel with no penalty because the guest mentioned the possibility of breaking a house rule and they never got back to me. (This only happened once with an IB - and Airbnb was prompt in cancelling)