Refunds are governed by the Guest Refund Policy. You should know it backwards and forwards since many CS agents don't, and you'll need to refer to it to advocate for yourself.
https://www.airbnb.com/help/article/544/what-is-airbnbs-guest-refund-policy-for-homes
https://www.airbnb.com/terms/guest_refund_policy
The guest needs to raise an issue within 24 hours of discovering it. If the issue isn't raised DURING the stay, they don't have much of a leg to stand on in terms of refunds after the fact.
If someone, during their stay, starts down this path I'll give the benefit of the doubt it's an honest complaint. If they find a bug or dust bunny under the bed, offer to re-clean. If they refuse, that says a lot.
If they keep pushing for a refund after you've addressed the issue, I'd choose to call Airbnb at that point and ask that they're re-homed, as evidently what we offer isn't a good fit for the guest. It's kind of a power move, since you're calling the guest on their BS and forcing them to interrupt their stay to find another place. The guest will be refunded any un-stayed nights, so be prepared for that.
If you keep playing the game, address/fix each issue to the best of your abilities as they arise. Each time refer back to the guest refund policy. Is this something that constitutes a guest travel issue? If yes, did they give you a chance to resolve it before involving Airbnb?
I've personally not dealt with someone doing this (finding tiny things to claim refund about), although I did have one guest attempt to cancel on arrival because the place was too small (she hadn't read anything). Knowing and referring to the guest refund policy saved my bacon. Air upheld my refusal to refund her stay.