An inelegant (but effective) method of preventing F8
access in this scenario is to modify the keyboard so that
F8 is not usable. You can do this either by placing it
within an enclosure that prevents someone from getting to
the key or, you can pull the keycap and clip the lead for
that key then replace the keycap. Assuming that someone
of ill-intent doesn't hook up a different keyboard you
are good to go and you don't eliminate your ability to
administer the system.
It is very difficult to completely secure a system to
which you cannot prevent physical access.