According to this thread: http://n4.nabble.com/command-line-shutdown-td367537.html

It's possible to trigger Hudson shutdown by visiting http://server/hudson/exit while being logged as a user with necessary admin privileges. It seems that there's no link to this location from the "Manage Hudson" panel - one can only click "Prepare for Shutdown", but not shutdown.

As to the location of the shutdown link, I think the user would expect it to be at the bottom, below the "Prepare for Shutdown" which is the last one currently.