Today i had to restrict an user's configuration and access abilities on an Ubuntu 8.10 System.
Beside the settings one can apply in Gnome's user account configuration there are several tools to customize more aspects of system access:
These four are coming via apt-get. One'll be able to lock down the system to a kiosk mode with this tools.
Another fine approach is to set the shell for an user to screen command or disable shell access.
More on this here...
Do you've futher proposals? Just drop a comment here.