There are two ways to do this:
1. Got to "Rates" set your rates to end at the end of the season. Create new rates starting at the start of the new season.
2. If you have many rates it is easier to set an exceptional period. To do so go to "Properties" click on the setup icon go to "Booking Rules" and scroll down to "Exceptional Period Booking Style" enter your closing period and set it to "Blackout Period - No new bookings accepted.