You really should stick to your cancelation policy, it’s there for a reason, whatever your reason may be. For me, it’s because great care and time goes into doing the little extras before each guest, on top of the cleaning and preparing that you do no matter what.
If you do decide that you would like to give them something back, make sure they are the ones who cancel - do not cancel it for them - this will penalize you and cost you in the future. Once you receive their cancelation notice, you will see an option to refund any amount or nothing at all.
Good luck!