
To disable Rosetta, simply run the following command in Terminal:
sudo sysctl -w kern.exec.archhandler.powerpc=/usr/libexec/oah/RosettaNonGrataTo re-enable Rosetta after it's been disabled, just run the following command in Terminal:
sudo sysctl -w kern.exec.archhandler.powerpc=/usr/libexec/oah/translateThese commands set a system environment variable that tells the system which program to run when you try to launch a non-native app: to ask you to install Rosetta, or to run it using Rosetta.
[robg adds: I tested these commands, and they work as described.]