A genre nazi will argue against anyone who claims the music in question is a different genre than what the grammar nazi has said it to be.
The word "grammar nazi" stems from the slang term for someone who aggresively corrects people on their spelling, vocabulary and grammar - a "Grammar Nazi".
"Deadmau5 is a progressive house/trance artist, not trance/progressive house artist, you idiot!"
"Metallica are a thrash metal/speed metal band, not a speed metal/thrash metal band, you idiot!"
"Rihanna is a R&B/pop music singer, not a pop music/R&B singer, you idiot!"