You can take care of this yourself by clicking on the "Report a Problem" and then submitting a claim through the resolution center. Anything you can provide as evidence (photos, receipts, etc) should be attached to help support your case. If the guest disagrees, Airbnb will intervene in deciding what to charge the guests.
Hmmm...I just checked your listing and it doesn't look like you have a deposit! I urge you to do so - if you don't have one, Airbnb can only charge if the guest agrees to the charges (at least that's how I understand it).