Dealing with the dangerous ones, eh?

The trick is to have the security, but not blatantly issue a challenge.

Theres a lot one can do to safeguard their system, but it varies depending on the OS.
What kind of OS will you be working with?
Does anyone else use this machine besides you?

Another universal solution is to just unplug the NIC cable (assuming you dont need network access & you'll still want that external drive).

~DM