How to prevent app from auto starting
There are two places for LaunchAgents to be called from:


/Library/LaunchAgents - this launches a process at login for every user
~/Library/LaunchAgents - this launches a process at login for a specific user.


The Spotify startup plist is found in the latter; the com.spotify.webhelper.plist allows you to open Spotify via the web.

Issue the command launchctl list | grep -i spotify and you should find (up to) two plists that are loaded:

$ launchctl list | grep spotify
-       0   com.spotify.client.startuphelper
9087    0   com.spotify.webhelper


To unload them, you can uncheck the option in Advanced Settings
 (Command,)



Or...

You can issue the command:

$ launchctl unload com.spotify.client.startuphelper.plist