Either Air is blocking those dates like they did throughout all of last year, or they think there is something wrong with the guest (not ID verified or CC payment issue).
We do not allow Air to refuse a Guest for nothing more "a stay happens to include Halloween". Or any other holiday. It is our place and our decision.
Have the guest book some other date with you, well past your cancellation policy. Now you can see who they are and if you like them as guests. You also have their contact info and can make arrangements as you see fit for the dates that they want.