Your statement, you do not know the importance of the reason people are in town. I usually do not ask that, but it gives more legitimacy to their visit.
But, although I have a whole litany of house rules, in particular no smoking and no pets–I have had people come who tried to smoke. I have signs all over the place, even in a photo on the main page of my ad.
Before I approve I sent them a message saying, "I presume you do not smoke or vape and do not have pets. We live on this property and both my wife and son are extremely allergic, ignoring this house rule could end up putting one of them in the hospital. Please confirm your acceptance of this policy if you want me to approve your reservation.