securing an Ubuntu box in a shared office?

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

Re: How to look up the serial port in debian?

> Could anyone show me how to look up the used serial ports in Debian? I want > to use a serial port terminal to connect to a target board. But the > terminal said that the port I chose was already in use. So I came to ask > for help. They're usually on the back of the box. Here's what to look