Take 'em to court. That's your next step if you want to pursue it further.
I always shoot video of the state of my guest suite before guests check in. At the start, I video myself announcing the date and time, just in case a guest tries to argue that it was old video. I also shoot video of the sticky note that says "Welcome [name of guest]" that I leave on the door. With video to prove your case, even Airbnb is not going to be able to rule against you.