Far as I am aware, all they did was unfollow everyone they were following. Which they did not have to do – they could have accomplished the same thing (getting rid of too many email notifications, so far as I am aware, was the motivation) by simply adjusting their preferences.
But of course if you want to cherry pick out the 'haters' then you can delete everyone you're following.
As for deleting everything and starting from scratch, it looks like you need a tool like this:
http://deletealltweets.com/connect.php?hl=en (I have never used that tool and never heard of it until Googling it just now, so I can't comment on its effectiveness).
They are currently (as of the writing of this post) following 88 accounts and are followed by 109k.