How to delete iTunes once and for all from the command-line?
I haven't tested this yet, but softwareupdate seems to support adding ignored updates:

softwareupdate --ignore iTunes

--ignore ...
            Manages the per-machine list of ignored updates.

--reset-ignored
            Clears the list of all ignored updates.

[...]

sudo softwareupdate --ignore iCal iPhoto

      Ignored updates:
      (iPod, SafariUpdate)


This might prevent iTunes from getting reinstalled when you upgrade OS X:

sudo rm -r /Applications/iTunes.app
touch /Applications/iTunes.app
chflags hidden /Applications/iTunes.app