Some fans take it on themselves to police "true" fandom, weeding out the heretics so that only the faithful in the Church of SciFi (or whatever) remain. They obviously do damage to fandom in general, their chosen obsession in particular and to authors and creators everywhere, at the same time as thinking either that they are doing someone a favour, or perhaps that they are somehow "winning" at fandom.
These are nerd bullies. "Oh, you can't name every actor who played The Doctor's companion? What makes you think you're a true fan, then, eh? Are you gonna cry now?" That's really the only motivation I can think of that makes sense. It's not a good motivation, just a motivation. These people may have been bullied at school for their unusual tastes, and when they find themselves in a place where everyone is a lot like them (a convention or just a fan subculture), their misplaced rage and a sense of localised superiority makes them lash out. Finally they can be the tiny king of this tiny little hill!
They're not going to be put off by the creators of their chosen show/movie/book/comic saying "cut it out", or at least not permanently. They need to heal.
The only other reason I can think for behaving this way is when you think someone is faking as a publicity stunt or as a way to sell something, like someone putting bad perfume in a turtle-shaped bottle and wearing a turtle suit to sell it to fans of the Ninja Turtles. That person may be pretending to be a fan, but the best way to get them to stop is to pay them no attention or money at all. Maybe what they're doing is damaging to the "brand" of the fandom subculture you're in, but being a bully about it is damaging too. Walk away, let their parasitic business fail and keep loving what you love.
Mokalus of Borg
PS - True fandom is gleeful and positive and inclusive.
PPS - A true fan just wants everyone to experience their joy.