In GMail, you can log out other sessions if you're logged in. At the very bottom, it says something like "Last account activity: xx minutes ago ..." followed by a Details link. Click it and you'll see the button to log out everyone else but your session.
Otherwise, I don't think so and am watching this thread to learn.
I'm sorry, after rereading my post I think I might not have been clear. I was asking specifically about HN cookies.