Even if the the subject says no spoilers, if a thread is about a book I don't want a spoiler on I don't read it on the assumption someone may cross a line or give me more info than I want.
I'm not normally one to blame the victim but if someone is complaining about a spoiler post I ask myself a lot, "why are you reading a thread about a book you haven't read yet if you care that much about not reading spoilers?"
If the thread is not labeled as (Spoiler!) and there are open spoilers for the book(s) in the topic, someone who hasn't read the book could be spoiled. Also, I don't want to see open spoilers for other book(s) not specifically in the thread regardless of (Spoilers!) or not.