The earliest and latest booking times in your account are default set between 7am-7pm to ensure a user in your account does not schedule a message by mistake for say 3am instead of 3pm.
However, you can change the time settings here: CONTACTS > SETTINGS > Contact List > Earliest Booking time & Latest Booking Time.
NOTE: This option will however not stop messages if the user books an immediate message or sends messages to recipients manually.