This is most annoying.
When a guest is attempting to book, why does their lack of fiscal responsibility and soundness result in my calendar being blocked? No one holds my tickets, shopping cart, seat etc., when I buy something online with a credit card that doesn't work. Why should my calendar be blocked for 24 hours while they straighten out their mess?
There should be a line in the reservation request informing hosts of this before we agree to accept the request.
"You just got a request for <dates in the future>. YOUR GUEST'S PAYMENT INFORMATION HAS NOT BEEN VERIFIED. Accept if you can host, or decline if these dates don’t work for you."