I know I've already posted in this thread, but this same topic came up in another thread recently, so I feel the overall issue needs slightly more explanation.
If the dialogue or scenes are word-for-word copy-pasted from another author's story, then report them for plagiarism. Ff.n takes the issue of plagiarism very seriously and the offending story will be deleted almost the same day as your report. I know this for fact because one of my own stories was plagiarized by another author recently. We had completely different premises, but entire blocks of his narration were taken directly from my story -right down to the typos. I reported the story and cited our posting dates in the report (mine went up two days before the copy-cat's did), and within a matter of hours the story was taken down.
So, if these authors are flat-out copying. That is plagiarism. That is reportable. FF.N will delete them.
But if the scenes just sound the same, but the author(s) use different phrasing/wording to describe what's happening or how the characters are feeling/reacting, then I'm sorry, but that's not copying. That's regurgitating the same information but in their own words. Its no different than in school when they ask you to answer questions from a textbook. The teacher doesn't want you to copy the text from the book, they want you to give the same information but in different words. Yeah, its irritating when reading fiction (not just fan fiction, but professionally published work as well) to read what feels like the same scene a million times just from different authors. But that doesn't mean any one of them stole the scene from anyone else. Depending on what genre a person is writing for, there are just some scenes that will ALWAYS happen no matter who is writing it.
You cannot "steal" an idea. You can only "steal" text.