After rereading, I'm starting to understand.
What is fair?
You may think that because they did not comply with checkin requirements, you should get their money .
They may think that because they have already paid you, they can come whenever they want.
The well has been poisoned. You don't want them and they don't want you.
The easiest thing is just to back out of the agreement and leave both parties unharmed. Give them their money back.
On the otherhand, you are harmed, because you cant rent the space in such a short time.
I would ask Airbnb. They are a 3rd party with experience with this situation. They know the track record of these guests. It is why we pay them.
Even if you are unhappy with the result, the decision has been made and you can move on.