The alternator needs to be switched off also to stop spark and fuel pump if the engine is running.
If you cut the battery out of the system, the car (and by virtue the alternator) will never see any power.
With no spark or fuel, I can tell you the engine won't be running. Are you wanting to turn the car on, then cut power to the alternator and have everything run off the battery? Maybe you should explain a little more what it is you're trying to accomplish...
"Give a man a match, and he'll be warm for a minute, but set him on fire, and he'll be warm for the rest of his life."