@Valerie579 you can call Airbnb and explain the situation, maybe they will take it down as third-party bookings are against the rules and the guest never saw your place. But... you kind of brought this on yourself.
Next time: if your guest wants to stay additional nights, they should request an alteration of their reservation within the Airbnb app or site. Then you will have the option to accept it, at which point the nights will be blocked and payment will be automatic. (Or you can initiate the alteration and they can accept it, same result.) If another guest books before that happens you should honor the new booking.
And you can always block dates in your calendar.