Hey Carol - I would suggest that the 'service dog' situation is not always as it is presented, so you may want to do alot of chitchat to flesh it out...unfortunately, ABnB will state that you must accept them or potentially face sanctions and/or possible lawsuits...the 'comfort animal' frenzy and the ADA prohibitions for requiring certifications/proof of medical needs does make it difficult to weed out the chaffe.
I personally feel that I have to consider my overall business - unlike a hotel that can dedicate a block of rooms for folks with animals, I have only a single space to let - if/when I am forced to allow a service dog in then I no longer can say that I am 'animal allergen' free - I do my own cleaning assisted by my husband, and he is deathly allergic to dogs; I lose that backup assistant and must farm out my cleaning needs for the rest of my renting days!
All that being said, I recommend that you charge a must heftier cleaning fee for any service dog that you approve - my cousin approved a service dog and a huge pitbull-mix show up to stay in her bedroom for a whole week - the amount of cleaning to remove the smell was very tedious and time-consuming, to a point that the next guest had to be moved to another accommodation.
Charge well extra for the dog accommodation, and when the 'service dog' comes and goes without issue or extraordinary cleaning requirements you can always refund a portion for the guest's courtesy of being mindful and cleanly with their dog...and keep that potential refund under your hat, otherwise you'll be arguing with the guest forever as to whether or not they deserved it or not...just sayin...