I have just received an instant booking for dates when we already have guests. It's clearly stated on our calender that it's full, but Airbnb accepted the booking.
Call the company. This has happened to others on this site in the past. They will cancel for you with no penalty, and will then help the guest find another space, which is nice, as it is not their fault.
855-424-7262
I have had to call twice, and I have gotten an actual human within about 5-7 minutes.