Yes you can. You can decline any booking requests that you can’t accommodate for, but you should do so within 24 hours.
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've declined, ask the guest to send a new request or send them a special offer :-)