You just have to be fair about it.
Try to stay with things that happened concurrently (pace the show).
If you have a spoiler, even with tags, if it is important to the central plot BE VAGUE. Those that read it already will probably be able to figure it out (or PM you) and the others, if they ran over it despite the spoiler tag, will still have some mystery.
As an example: whats-her-face doing it with whats-her-face is not really a "spoiler", it is more like a tease. But if you were to talk about Davos and the raid before it happened then you are ruining it for some people.....
As for you people whining about "post it somewhere else"... there is nowhere else. The show keeps people interested, so there is new stuff to talk about. Not many post about the book ANYWHERE. So be fair to the bookies as well. So long as they are not giving obits on each of the characters....
*shrug*