@Susan_Armstrong -- are you getting Inquiries? Or actual Booking Requests? As mentioned, declining an Inquiry does not affect your ratings or anything, as long as you answer the first message.
Asking about unavailable dates indicates Newbie guests, who really should be educated by you (the good Host) -- all you need say is "those dates are unavailable", or "if you can't see my place on that date i means I'm already booked". A simple response is all that's needed.
People who are already booked, should be contacting you through the AirBnb messaging system, not an Inquiry. If they are using an Inquiry instead of the messaging system, again this indicates Newbie Guests, and again this education is something you should be doing as a good Host.
Booked guests certainly expect you to answer some questions about your listing; but more than one or two questions should be answered with "all of that is already covered in my listing description on AirBnb, please re-read hat is written there..."