Closing a board
Close a board by selecting "Close Board" from the board menu (open the board's menu and click the "More" button to expand the menu options). You have to be an admin of a board to close it. Other board members will receive a notification that the board has been closed.
Close a board from the board's menu.
Once a board is closed, it is no longer visible in your Boards list and can only be accessed by clicking the Boards button at the top left of your Trello page and selecting "See closed boards..."
If you're trying to leave a board after it's been closed, you can do so on this page.
(Not an admin and trying to leave the board? See Removing yourself from a board).
Note: Business Class team admins are able to close any board within their team, even if they are not an admin of that particular board or if it is a 'private' board within the team.
Closing a board on the mobile app
If you're using the Trello app for iOS or Android, you can close a board by first opening that board and then opening it's menu (the "..." icon in the top right of the page). From there, select the gear icon in the iOS app or the "Board Settings" option on Android. Select "Archive Board" from the options in the settings to close the board.