While it's important to respond to all the booking inquiries you receive, you can tell any guest that your listing is unavailable for a tour they've asked about. What's most important is that you respond within 24 hours. Responding quickly will preserve your response rate.
If you decide to let a guest know that your listing is unavailable, Nonlocal sends them additional listing recommendations so they can contact other guides. That's why it's important to let guests know that you can't tour with them—it will help them make their plans, and they'll get added support from Nonlocal.