@Kathy286
From what I;ve always understood, if you decline a booking giving the reason that the dates are unavailable, then the dates will get blocked, but you can go into your calendar and unblock them. If you give one of the other reasons, e.g. the listing does not suit the guest's requirements, the dates aren't blocked. I've not done this for a long time so I don't know if that is still the case or not.
The reason I haven't done this in a while is because Airbnb started measuring hosts on their acceptance rate, which was not a criteria when I started. Now, I avoid hitting the decline button if I can. I firstly try to get the guest to withdraw their booking request. If that doesn't work, I would call up Airbnb and explain the situation.
I did this recently with a guest who tried to book for themselves and their child when my listing clearly states I don't accept under 19s. Even if I did accept them, they were trying to book for one person when it should have been two as the child is over 12. I explained to the guest, who was totally unresponsive. When time started running out, I called Airbnb and the rep cancelled the request from their side.
So, it worked but really, as hosts we should absolutely not be put under this pressure, nor have to spend so much time dealing with requests from guests trying to book inappropriately. Not having availability when your calendar shows you have is quite another matter. It is the host's resposibility to stay on top of the calendar.
@Julia1Have you tried using Rule Sets? Perhaps there is an option to customise check in times for specific dates.
Also, is this a new thing, but I just got a notification about a pending request (I'm waiting for the guest to respond to my questions as they seem to be under the impression they can check in/out at any hour) that says if I don't accept or decline in time the dates will be blocked. I have never seen that before. Airbnb seem to be coming up with more and more ways to penalise hosts...