If you are OK never updating your OS and/or repeating the 'sudo mv' when the program gets patched, I would do the following:
- Make a Time Machine Backup (or whatever else backup you use to protect yourself from a bad sudo command)
sudo mv /Applications/iTunes.app/Contents/MacOS/iTunes /private/tmp
This leaves the helper app, the libraries and the rest of the app around, but relocates the actual executable program to a place where it won't launch. You can then reboot and test to make sure that the move was effective and your Mac works otherwise.
The bonus of this is you can move the binary back (from /tmp or from Time Machine) when needed and get a fun message whenever anyone tries to start iTunes on your Mac:
2/24/14 12:25:44.309 PM com.apple.launchd.peruser.501: (com.apple.iTunes.47376) Job failed to exec(3) for weird reason: 2
This one command to move one file is quite safe and doesn't have any large bad effects that I can see and I have used it from time to time to disable iTunes in lab situations where we didn't want iOS devices being managed or music being played.