Click on the Gear icon in the upper right-hand side and then Advanced Settings.
Enter a number into the field labeled "How soon before an activity or rental should we stop taking bookings?"
IMPORTANT: This setting refers to either minutes, hours, or days. For example, enter 48 hours to stop accepting ONLINE bookings 2 days before the activity start time. Setting the cutoff time to '0' hours will allow customers to book up to the time of an activity.