Academics tend to whine that whatever books are selling more than the books they want to have people read will ruin literature FOREVER. Usually it's the fault of academia in the first place, when they write and print books only for the wealthy and privileged then complain about the unwashed masses not reading the "True Art" books. Some good examples, look at the reviews of True Art academic literary critics for Wilkie Collins' The Woman in White, often considered now to be a great example of nineteenth century literature, and The Lord of the Rings, whose reviews by the literati were not only absolutely disparaging but in many cases were by people who plainly hadn't read the book.
How often does one get Death of the Author literally as well as figuratively, let alone simultaneously?
A Series of Unfortunate Events is Ruined FOREVER at the end of the thirteenth and final book know as The End when not all the mysteries of the series were solved that were introduced at the end of book five The Austere Academy this includes the fates of the characters.
The Elder Scrolls novels ruined The Elder Scrolls FOREVER! On one level the novels were announced the same day Bethesda said that the next game in the series was a long way away, so it could be Pavlovian, however people complain about things in the book, claiming they ruin the series FOREVER, even though those things were major parts of the games, especially Morrowind.
Every time anything happens in the Warrior Cats series (which is all the time), at least a quarter of the fanbase is going to scream this.
To be more specific, some "fans" insist the series was Ruined FOREVER at the beginning of the second arc, and some believe it was the middle of the second arc, and some... Ah, forget it. The series has apparently been Ruined FOREVER at the beginning, middle, and end of every arc except for the first one.
Twilight. According to some people, the series's existing means vampires (and maybe literature) are ruined FOREVER AND EVER.
Note that exactly the same complaints were lodged when Anne Rice's vampire literature became popular as well.
An interesting point for non-haters and haters of Twilight alike, they seem to agree that Breaking Dawn ruined the series forever.
The Heroes of Olympus has a vocal minority of people who believe that the revelation that Nico di Angelo is gay and has feelings for Percy for most of their acquaintance ruined the series, saying it had no other relevance than to be part of the author's liberal agenda and is inappropriate subject matter for the target audience.
V. C. Andrews fans have a few different opinions on this. Either when the real Andrews died and Andrew Neiderman took over for the books, after the Casteel series ended and Neiderman began the Cutler series (The first series not started by Andrews herself), after the publication of the Orphans miniseries with the change in format, when they did away with the stepback covers, or when Neiderman stopped doing family sagas all together.
Dollanganger fans had a field day with the Christopher's Diary series, and the reveal that Cory was alive.
With the publication of Harper Lee's "lost" novel, Go Set a Watchman, a sequel to To Kill a Mockingbird the transformation of Atticus Finch into a bitter bigot trying to stop the desegregation of the Deep South has already tarnished the novel's legacy.