have them contact airbnb. This is not your issue. If you declined the inquiry, then their credit card was NOT charged, so I am not sure why they are telling you this. I am a host AND have personally booked my own stays thru Airbnb and nothing is charged until the Host confirms the booking. If you didn't confirm a booking, then they weren't charged. Again, have them contact Airbnb directly. They have the same HELP button on their account as well. Good Luck
If you feel that me or another host have helped you, feel free to click on the "thumbs up" button at the end of any post. Thank you so much.
Aloha, Momi
Great way to contact Airbnb or via Twitter at AirbnbHelp / Facebook