@Anonymous You can try contacting AirBnB, pointing them at the chain of messages between you and the guest (only if the messages were all exchanged within the AirBnB message system [advice: NEVER communicate in any other way]) and see if AirBnB has any sympathy. If they don't, do two things: First, chalk it up as a learning experience, and then go read pretty much every inch of AirBnB's Help system and policies, so that you have a better understanding of what's going to happen with all of the many things that can go on within AirBnB.
A third suggestion: drop into these community forums every now and then, read about issues other hosts are having, and read the explanations, suggestions, and solutions. That's what helped me avoid a hundred pitfalls.