Hi @Barry48 you have to get in front of this. Contact the guest in the airbnb messaging system and write something like "Hi X, as we've discussed, I have offered to send a cleaner over to reclean the apartment. You have indicated that you don't want this. I can come over myself to inspect the areas that you don't feel are up to standard, what time is good?"
I'd go over myself if possible, as some guests are very easy to be pushy because there is no human contact. But if you go there and express real concern, listen to the guest, and try and diffuse the situation, it might work. Whatever you do, document all your actions in the message thread. The first thing that Airbnb does in any dispute like this is go to the messages, and see what is the dialogue. If there is nothing in there from you, the airbnb support might assume that you've taken no action at all, and could decide the guest is right. So you need to make sure that you have shown that you've done what you can to fix the situation, and then it will be less likely that the guest will get their way. They are obviously just looking for a free ride if they want a refund, but want to continue to stay there....
Good luck!