You can choose between different cancellation policies (flexible, moderate, strict). If you have a strict or moderate cancellation policy, then the guest will be charged some amount of the money depending on how close to the date it is. I believe (but am not positive) that flexible is 100% refund up to 24 hours ahead of time, moderate is full refund up to 5 days ahead of time, and strict is 50% refund up to one week ahead of time.
I do believe that you will get fewer bookings the stricter the policy, but it might be worth it if you would be losing a lot of money otherwise.