To clarify my position:
I've read all the books. I resent that "all the book-readers" part, I never spoil anything. I occasionally have to tell certain friends who haven't read the books to stop talking to me about their theories regarding the future of the story, as I don't want to give anything away. (I think my poker face is fairly decent, but these guys know me quite well, and better safe than sorry and all that.)
The issues raised by this teenager are twofold: smugness/feelings of superiority by people who've read the books, and purism/elitism.
Regarding the first one, I want to stress that this is hardly unique for Song of Ice and Fire/Game of Thrones. I experience this to some degree pretty much every time I watch a film or TV-show for the first time with someone who's already seen it, or when the positions are reversed. (It's often fun to see peoples' reactions to things as a certain genre of youtube-clips has shown). It's just happens a lot regarding this franchise because of how many people watch it and how many people have read the books. I think it's just a basic and common part of human nature, and while it can be managed (in short: people can try not to be d*cks about it), I don't think it will disappear completely any time soon.
Concerning the "purism" issue, I get really annoyed with how often I'm accused of being "purist" when I dislike alterations in adaptations. For example, most times Jackson parts with Tolkien, I side with Tolkien. This is not because I believe all changes are bad, it's just that Tolkien is usually a better storyteller than Jackson. And the same is the case with Game of Thrones. The times I think it's a shame they've altered things, it's not out of principal against alterations, it's because I believe what's in the books work better. And I do enjoy some of the alterations, like the expansion of Margaery Tyrell's character, as has been mentioned earlier.
(My favourite example, however, is the Tintin film, as I grew up reading those comics, and ended up loving the film, even though the story was a blend of several storylines with many elements added and subtracted.)
Of course some people are smug and spoiler-happy, and of course some people are purist. And we all know that some people are jerks when they're online. But generalisations are hardly ever useful nor accurate, and usually only good for creating arguments. At least refer to "some people", or "this guy I know", or "some d*ck on the internet", or something along these lines, rather than "people who read the books" or "people who haven't read the books".
(Huh, didn't even realise I'd brought my soapbox before I was done...)