Pre-approvals don't block your calendar until a guest accepts and confirms their booking.
Keep in mind that pre-approvals only give your guest the option to book—just like you can send multiple guests a pre-approval, a guest may have sent multiple hosts an inquiry.
Pre-approvals expire after 24 hours. After 24 hours, the guest can still request to book your listing at the price offered during in your pre-approval, but you'll have to approve the request.
You can also remove a pre-approval at any time from your message thread with the guest.