I've already tried messing with the task manager's startup tab, changing the Startup Apps in Windows 10 settings, and selecting no in Spotify's advanced settings. Spotify isn't even listed in the Task Manager or Startup Apps until I select yes in Spotify's advanced settings. Even when I select no, it still boots up.
I also tried deleting SpotifyStartupTask.exe from the AppData/Roaming/Spotify folder but to no avail. Maybe if I mess with the "prefs" file?
Any other suggestions?