@Stephen1412,
You have a few options. You can reach out to the guest, and ask them to give their side of the story regarding the other host's review. I'd probably call to speak with the person to assess their demeanor and tone (are they defensive, elusive, aggressive, or take ownership and apologize). If you live on site, you can reinforce this, and let the guest that you will be not accept any type of bad behavior. I once had a questionable guest, and I required that she pay a refundable $300 deposit via the Resolution Center. She wasn't perfect, but didn't do anything that warranted keeping any part of her deposit.
Of course, you can contact Airbnb, and ask them to give you a penalty free cancellation, because the poor review showed up after you accepted the reservation or it was IB'd.