Have fans ever ruined a game for me? No, not really; unlike a lot of people I know, I'm not going to just stop enjoying something because other fans are dicks/weeaboos. That does not mean, however, that I have never been embarrassed to admit to liking a game/movie/whatever fans might be into, as a result of a rabid fan-base. Allow me to explain:
I love zombies. I love Vampires. I love Family Guy. I love Final Fantasy VII. I love The Rocky Horror Picture Show. I love Halo 2 (specifically 2, don't really enjoy any of the others). Call of Duty: The Next One is generally a good time.
All of the things I have mentioned have had the chance to be ruined by their fans, for various reasons. Zombies, for example, have always been a favorite of mine... imagine my disappointment when they turn out to be some fan-boy wet dream. Vampires were nearly ruined by Twilight and it's ilk, and even before Twilight, there were the "vamp fags", yound people, mostly girls and gay dudes, who hung out at special clubs pretending to be vampires. Not ever LARP... just... why? Family Guy and Final Fantasy both have annoying fans, and when people think of Rocky Horror, they tend to think of hordes of men dressed as whorish women for novelty. Halo and Call of Duty need no explanation.
Still love it all, though. Why would I let a bunch of strangers decide what I should like?