Don't be ridiculous. There is no way for a show-watcher to know what is a spoiler or not, so they have to assume everything is. That's just how it works. Anything I as a book-reader say in that thread, whether I try to or not, is coloured by the fact that I know what is going to happen. If you say Tommen is next in line for the throne and they realize you've read the books, they're going to start asking themselves whether you're saying that because it's been mentioned in the show, you're just making an assumption based on real world succession laws, or you know that Tommen is going to be crowned King. Your post can be completely innocent and it's still impossible for them to know that it is.
The solution is simple. Don't post in the thread about actual plot stuff. Any ambiguity to that rule leads to the instances we've had of people actually posting spoilers, whether they meant to or not.