A script that waits patiently for iTunes to start and then shoots it on sight. This script can be easily modified to kill any other programs when they start too.
iTunes is annoying and I often open it up by accident.
On Mac, the easiest way to install this is with brew:
$ brew tap aviaviavi/homebrew-tap
$ brew install aviaviavi/homebrew-tap/sniper
If you'd like to have sniper run on boot as a background service, simply execute:
$ brew services start aviaviavi/tap/sniper
Or just grab the script and do your thing :)