If at all possible, ban spectate. The subject was bound to surface eventually, so I might as well be the one to address the elephant in the room.
It isn't necessarily a bad thing to spectate other teams' testing decks, but it is questionable or unethical to some.
Its not really possible. At best, for the next war, players can put in their application if they have alt account for testing purposes.
People use spectate outside the war too, and if you ban it with rules, I'm sure we'll hear accusations how someone used "perfect counter" just because they were spying on spectate.
To me, the issue is questionable; everyone has access to spec mode (and as heard on chat, lots of teams at least tried to catch someone testing), and everyone CAN use alt accounts if they have the will to farm with them. Still,
knowing* your opponent deck, is a bit detrimental to the event.
*You can never be sure that what you saw is THE deck. Me and Shantu did some false testing just for teh lulz.