Nancy, so sorry that you are having to deal with some sour apples. Clearly, THEY must do all the work, not you! to change things around, etc....
for the moderate cancellation policy you have in place, in Airbnb rules it says : If the guest arrives and decides to leave early, 50% of the accommodation fees for the nights not spent 24 hours after the cancellation occurs are refunded.
So had they actually booked for a night/nights 24 h AFTER they cancelled? Doesn't sound like it from your report that they even cancelled in the first place, so I would not even bother to give it any more energy or thought: let them work it out withAirbnb, you just sit tight... 🙂
Good luck!