How about being generous with Superglue on the PS/2 and USB connectors on the firewall? That way nobody can use a keyboard
To be serious, if you cannot prevent physical access by unauthorized persons, there is no true security. Even if you would disable console access, they still can press the RESET button, pull out the power cord, or change the disk or CF card.
If they take your disk out, put it in another machine, reboot it single user mode, they can change the root password, remove or change your SSH keys. If after that, they put back the disk, you have a slight problem