I don't really believe in this "spoilers here" thing. I mean, if you read it, you won't know it's a spoiler unless someone told you, right? You'd just assume that it's part of the story.
If it was just some fan-exchange, then that would be a completely different story. I heartily agree that spoilers should be marked in a fan discussion.
I just don't think it need be marked for fanfiction.
I suppose if you really want them [and my *cough* well-crafted argument wasn't enough], let me know, and I'll give it to you.