First you need to vacate each seat, then you can disable the board. It will keep all the info intact but will shut the board down.
To do this, go to the board an click Manage.
Use the action menu for the first seat and use Vacate Seat. Enter the End date (required) and the Reason (optional) the seat is being vacated and hit Submit. Now repeat this for each seat on the board. Once all seats are vacant, you can use the gray Action button at the top to disable the board.
Now at the top of the board page use the Action button (next to Edit Board) and select Disable Board. The board will show in yellow in the Admin Portal and be hidden in the Public Portal.