On the booking form, you can decide whether or not you want to allow your guests to choose their check-in and check-out times. By default, check-in and check-out times can be chosen by the guest. However, this can be changed.
You can access these settings:
ResBook > Settings 
> System Settings > General Settings > (Scroll to "Checkin & Checkout")
These two fields will allow you to determine the 'From' and 'To' times for check-in.
In order to prevent the guest from choosing check-in times on the online booking form, you must set both the 'From' and 'To' times to be the exact same time. This will prevent guest from changing the check-in times.