Alternately, perhaps if one wants to discuss a book without having to worry about being spoiled they should start a thread that explicitly requests that spoilers not be added. They may still happen inadvertently, but that's always going to be a risk.
Absolutely - if you want to discuss something but do not want spoilers, this is definitely the approach to take.
Another forum I read has a long running Game of Thrones thread, but because it is for the TV series it states in the thread title "No Book Spoilers". That could work just as well here. If you want to discuss a book in a series but don't want to risk spoilers of other books in that series (for example) just say so in your thread title.
People may make mistakes, but people here will try their best to honour no spoiler requests. If you think someone is posting spoilers deliberately then notify on the post - that's the quickest way to let me and the other mods know something's wrong.