after SEVERAL attempts at this annoying activity, i can confirm this works with OS X El Capitan 10.11.2 Thank you very much!
You can either:
1. Use an app to disable the iTunes play/pause hotkeys (which also disables this bluetooth auto-start functionality.
The app can be downloaded from this link: http://redth.codes/itunes-media-hotkey-disabler/
2. Make the same changes yourself using terminal:
To disable the functionality:
launchctl unload -w /System/Library/LaunchAgents/com.apple.rcd.plist
To re-enable it:
launchctl load -w /System/Library/LaunchAgents/com.apple.rcd.plist