Disappointingly, Compiz didn’t run well on my Ubuntu 9.04, giving me a black screen. Of course I couldn’t undo the Compiz setting in the black screen.
To undo the setting and deactivate Compiz, you have to delete .gconf/apps/compiz/ from your home directory.
First, kill X:
Login in the command prompt by pressing Ctrl + Alt + F1. Login in this screen with your username.
sudo kill `cat /tmp/.X0-lock`
Then remove the Compiz config:
rm -rf .gconf/apps/compiz/
Press Ctrl + Alt + F7 to go back to the graphical interface. You should have your non-Compiz desktop back.