securing an Ubuntu box in a shared office?

[Anonyma]

I know that anyone who can get into a computer can make it insecure
(by putting the hard drive in another machine or taking the mo/board
battery out to clear the bios password), but what are the steps I can
realistically take to make a computer in a shared office secure?  I
can only think of these two:

1. set a BIOS password

2. set a GRUB password so no-one else can boot it into single-user
   mode

Anything else?