Hello !
Sorry to hear that !
I just had the same problem with a guess pretending there was a umbearable bad smell in my renewed flat, asking for sure for a refund. 3 different people (including me) went in the flat and didn't smell anything, during 3 differents days during his stay.
Then he stopped to pretend anything about the smell, and start new claims about other things, like "the shower head fixing seat doesn't work", as my bathroom got a bath, etc.
He is a local resident, who was supposed to stay in my listing as he was hosting his family in his own flat. I guess his plans changed, because he came alone, without his wife, and started to ask a refund for irrational stuffs from the beginning.
He left yesterday, and feel really better about that.
This is an awful hosting experience, and a very unpleasent way to start the year !