Within each Initiative Setup, there is an option called "Redirection URL" that can be found Initiative Setup > General Settings. This option provides Admins the ability to re-route all End-Users to another URL rather than the Initiative.

This feature can be utilized in situations where End-Users should no longer have access to an Initiative, but instead be redirected to another Initiative/external page. Rather than informing all End-Users to go to a different URL, they can continue to navigate to the previous URL and Brightidea will automatically redirect them. Note: This redirection option will end the User's session and they will automatically be logged back in when redirecting to another URL.
This functionality is designed only for closed Initiatives and it will prevent all End-Users from accessing all pages including any Submission URLs.
Please Note: All Admins (including Moderators and Analysts) will still be able to access the Initiative, it will only redirect for End-Users!
when you say "all users", do the systems admin still have access? I suppose so.
Could this be set-up for when you change a campaign access from general population ( a large group) to an evaluation group (small)? I suppose not.
Yes, they have access. But in order to get back to the campaign, they have to navigate via Enterprise and turn it off if need be.
The admin can still go in, from navigating from home page. The admin does not have to turn it off to go in.
I like to save the URL for the Admin Setup page for a WebStorm before this is enabled. That is a good access point for an Admin after a redirect has been set up.