First, make sure your teaching schedule is always up-to-date in the Tutor app (you'll find the ability to edit your schedule in the "Me" section). This will ensure that you will not receive requests when you are unavailable.
If you see a request and you're not available during the time(s) listed, please pass the request and wait for the next one. The student will not be able to respond to you if you ask questions in your reply, and this will also cause the student confusion.