Making a cd .. Block on login to computer

Ok, so I have code on startup which asks for the username and depending on the username given you will have to give your correct password, how can I make it so that when someone enters their password it'll put them in a directory and lock them into it so they cannot cd .. out?

I'm using CraftOS 1.3

You would be better off making your own shell but you could overwrite shell.setDir.
Assuming ".." is referring to root:

local oldSetDir = shell.setDir --# Keep a copy of the old shell.setDir.

shell.setDir = function(path)
     local prev = shell.dir() --# The dir we're in right now.
     oldSetDir(path) --# Set the dir.

     if shell.dir() == "" then
          oldSetDir(prev) --# If the current dir is root, reset.
          error("Access Denied") --# This will make it behave like attempting to delete rom.
     end
end

Keep in mind that you can still access files in the previous directory like so: ../afile. You could override shell.resolve and shell.resolveProgram for that.
Also, your own program will also have no access to root, so use oldSetDir instead of shell.setDir.