There's no way to achieve that level of security, given the way the
Macintosh OS works on a normal Macintosh. Beyond locking up the physical
computer, there is no way to prevent people from turning their systems off
and restarting from a floppy.
One potential workaround would be to unplug the floppy-drive cable from the
logic board. This wouldn't stop someone who knew what to look for, but it
would get in the way of the less knowledgeable.