How to disable or password protect downloading, file transfer and installation option in ubuntu ? Or how to customize it so that the we cannot download, transfer and install any extra file after the OS is functioning ?
You can do this with user permissions. Just set up different groups, with different privileges, for each of the 'roles' you might want to assign people, and then assign their user account to the relevant group.
This answer shows how you can do it with a fairly simple GUI. Just install gnome-system-tools through the software centre or from the command line and you should be able to see how to set things up.