I think they need separate beta forums, different URL, maybe different forum accounts.
I had the problem in a past beta where I honestly though I was posting only in bullroarer forums but was actually posting something secret to the world. Same player names in the live thread as were in the beta threads, same background color, same look, and vaguely the same topic. One of my few demerits :-(
That won't help, I'm afraid. Almost invariably the unintended NDA violations posted in these forums are because people simply came here and saw a forum called "Helm's Deep". As long as there is a forum about the announced forthcoming expansion, there will always be people who just don't look around them. If you look at the post count of the accidental violators, they are usually very low, showing that they are just not forum-savvy.