A new feature has been added that allows you to require advance notice for registrations. This will prevent participants from registering within a set # of days from today's date, for flexible and package date programs. 

This setting is located under the date setting on the Program -> Information tab, simply expand the 'Advanced Notice For Registrations +' link. The default for all programs will be 0, meaning no advanced notice is required, participants can register same-day if they wish. Changing this to 2 for example, would mean that participants cannot register within 2 days of today's date.