@Piotr48 I hope your conversations with this guest were made on the Airbnb messaging system- that way if she tries to get a full refund for her entire stay, there is a record of her refusing to have someone come check it out.
As Andrew suggests, refund her the unused day, and hope she doesn't pursue a further refund. If she does, that would indicate that she planned these complaints from day 1 to scam a free stay.
And in the future, if a guest makes a complaint like this, as a host, you have every right to send someone over in an emergency situation (like if it really could have been a gas leak) or access the apartment to check out something that could be serious, with 24 hours notice. The guest doesn't have the right to refuse to allow a host to do this.