Yeah I think if the title says SPOILERS and I go in the thread and I am spoiled (even a major spoiler) then it is entirely my fault as the reader.
However lets say someone makes a thread that has nothing to do with that particular book but it comes up in the discussion. Then I think if you are talking about something significant in the book it should be in spoiler code. This is common courtesy.
Now as for review threads that's tricky. Again review threads could just be for new readers to see if they want to read the book. A good review shouldn't have spoilers. However there are also the fans who might want to discuss what they read. In this case spoilers should be expected. So what type of thread is it?