@Susan1802 only you can know for sure if the cleanliness recall was worthy or bogus. Sometimes with these types the best response may be "it seems like you'd be happier elsewhere" with a calm, unbothered smile.
Easy for me to say though bc in my place, the guests need a place to stay way more than I need their money. But if they've arrived then they're going to be allowed to review you no matter what else happens. And you are going to get to review them. So if you're stuck with a crappy review and a crummy guest then sometimes the only consolation left is to at least get paid. (and to review them too)
As an alternative to the discount, how about this: they had a problem and you fixed it, seems like you'd owe them money if they had a problem and you didn't fix it.
Also, to be very clear, complete and utter cleanliness is a non-negotiable. And my listing is a Barn.