Good form to not blantantly toss stuff out, but the people that have read the books are ALSO watching the series, and have a right to comment on it as well. If they mistakenly use the wrong tense when talking about someone, or laugh a little when a newbie to the series speculates about how Ned's reign as king is going to go (or whatever), who's really at fault?
The person who gave out the spoiler. They are at fault.
There are plenty of posters that read the Doctor Who spoilers, but I can still participate in an episode discussion without worrying about learing what's going to happen in the next episode. I can't do the same in these threads.
If you're THAT anxious to avoid any possible hint of a spoiler, probably best to avoid any threads at all that pertain to it until you're caught up, accidents happen. Why is it on everyone ELSE to protect YOUR enjoyment of something? Best effort and all, but ultimately taking a risk if you jump into something late and insist on discussing it online.
In this post you've made the argument that people who've read the books should be able to participate in the discussion threads (and I agree with that), and that they should be forgiven for the regular spoiler dumps we've seen (which I disagree with), and now you're saying that people that haven't read the books shouldn't be able to participate in the discussion threads of a TV show they're watching. Really?
How about we do something simple? Why don't book readers who want to throw out spoilers participate in the thread that was created specifically for them?