We're not talking about the book. We're talking about how events from the book are going to be translated to the screen. So talking about it in the book thread wouldn't make much sense. The mods haven't seemed to have a problem lately, since the majority of the posts in here are people discussing just that, how their favorite events of the books have been, and are going to be translated on screen.
Were this a situation like The Walking Dead, where the TV show had branched so far away from it's source material talking about the books is meaningless, I would understand. But it's not.
And as far as I've seen, people have been respectful and posting spoilers from the books in tags, so I really don't see what the problem is. But if people really have a problem with it, I would suggest creating a separate forum dedicated to discussion of the show for those who have read the books. Since, again, people aren't discussing the books, they're discussing the treatment of specific events and speculating on how the show will handle them.