Put in your House Rules something like:
"Note that the drive to the property is steep, rocky, and will require you to go slowly or rent a vehicle with proper clearance to navigate to the top."
This way you put the burden on the guest, they accept that one of the "rules" is to provide the proper vehicle to get to the property so you don't have to negotiate with them after they've rented a tiny car. I would also post several very clear pictures of the tracks leading to the property.
It's not clear if Airbnb would have jumped in to re-assign these guests to a different property and consider it an extenuating circumstance. You were kind and acted exceptionally to give them back their money.