Dear Aunt TUAW,
Good lord, why does Spotify want to open .m4v files? And how do I make it stop that?
Your loving nephew,
Eh? What was that? Auntie was just giving the kids some Werthers in exchange for them agreeing to play quietly outside the house. Spotify? Good heavens! *fans self* What is this world coming to?
Spotify must have upped and changed a file association. Easy to fix, though.
Select any .m4v file in Finder, then choose File > Get Info (Command-I). When the Info window opens, locate the "Open with:" section. Select another application, like VLC or QuickTime Player, and then click Change All... This tells OS X to use that application to open all m4v movies.