You can not. Technically, since the guest did not expect or ask for a refund, they are still "renting" it. That is to say that they did not cancel, nor was there a change made to the reservation, so Airbnb will not consider them checked out until the time noted in the reservation.
You are being paid for the night, however, so no problem. You can not effectively double book (and be double paid) for the night, even if the guest is not actually sleeping there.
What about a situation where I tell a guest they can't get a refund, unless I can fill the empty dates that they are making available?
Can they amend their dates, but me not pay a refund (and me do it separately once another booking is made)?
This is exactly my situation.. No refund required or requested, guest requested to change the dates.. but airbnb system said that in accepting the change, I am accepting the refund. Is this because the guest requested the refund at their end (when they shouldn't have) or is this the way the ABB cancellation system works?