An alternative to fully shutting down the site would be to "freeze" it. The site is available, but nothing changes. (This could even happen 24 hours per day: updates only happen every 6 hours or something.)

To me, the alarmingly addictive thing about HN is that something interesting could happen at any time. Removing this real-time aspect, HN would still be interesting, but no more addictive than a good issue of the New Yorker or whatever.