@Nicky182 sigh. Some people have no common sense.
I would probably try to come to an agreement with the guest as ABB will likely side with them if they file a claim. Maybe half back? Just apologize that they couldn't enjoy their time in your space and say you are refunding them half of their stay cost as a courtesy. Don't ask them if its okay-- just do the refund. Then if they run to ABB at least it looks like you tried to sort it out.
Sadly, you may also want to consider adding a line to your house rules or description stating that you are in an agricultural zone and periodically guests may experience rural realities like roosters crowing, animal and fertilizer scents, insects, etc. Try to cover as many possibilities as you can think of that a guest could use to demand a freebie. This is the only way ABB will possibly side with you-- if you have disclosed it in your listing and guests book anyway. Good luck!