I am trying to lock down an Ubuntu 10.04 desktop environment for students. One issue I am having is that if a student clicks on a file type that is unknown, he can then click the 'select application' button and be given a list of applications including applications that I am trying to restrict him from such as remote desktop and others. He can then select a restricted application and even though it may generate an error, the restricted application continues to run for him to use.

I am using sabayon to lock down the desktop environment.

Is there any ideas out there, or maybe a way to remove that 'select application' button, or limit the list of applications that are displayed for restricted users.

Thanks in advance for the help.