meh, its not as bad as saying that Dumbledore dies just a week before the book where he actually dies is released, as I did at school, didnt know where to hide when I actually reached the page where it happens...
anyway, shouldn't be considered a spoiler, as its there for anyone who has read the books. True, some people are waiting for the series, and it might be not the best thing to do, but would you do something if someone spoiled now the batman film, for example? I have not seen it yet and I could say its a spoiler and he should be banned for it, but its stupid imo. (or at least thats what my common sense says...) Its out there, and has been out for a few years already (in the aSoIaF case), so you've had plenty of opportunity to do it.
and to ban someone for discussing the fifth book of the series, in which the whitewalkers do this or that and you thought i was going to spoil it and started to freak out..., that should not be bannable imo...
of course if he has repeatedly done it, its a different manner