You can decline any booking enquiries or reservations that you can’t accommodate, but you should do so within 24 hours. Whether you accept or decline, the amount of time it takes you to respond to requests is factored into your response rate.
When you decline a reservation request, you can choose to keep the calendar dates blocked or make them available for someone else to book. Update your calendar regularly so you only receive reservation requests for when you're able to host.
If you change your mind and would like to accommodate a reservation you declined, ask the guest to send a new request, or send them a pre-approval.