Knowledgebase
How to disable your phpBB3 board
If for some reason (e.g. moving your forum to another location, etc.) you want to make your phpBB3 forum unavailable to users, you can disable the board from the administration control panel.
To do this, log in to the administration panel, then on the home page of the admin panel (General tab) click on the Board settings link on the left. On the Board settings page mark the button Yes for the option labeled Disable board and click on the Submit button to save the change. In the text field next to the Disable board setting you can also type some brief text that will be displayed when somebody visits your forum.
Guests and registered users won't be able to view the topics or create new posts while the board is disabled, but as an administrator you'll still be able to access the administration panel and enable the forum whenever you want to do it.