We've just rolled out a a new feature that will allow you to block bookings beyond x days into the future. So for example, you can limit bookings to a maximum of 6 months out from the date guests are making their booking.
This setting is located under the date setting on the Program -> Information tab, simply expand the 'Rolling booking window +' link.
Important note: ALL flexible date programs will get a default future booking window of 365 days, meaning guests can always book a year out. If you would like to provide a longer booking window, you will need to adjust that manually within each program.