Planning on limiting signal using a physical mechanism of some sort's
just
a little too scifi to be useful.
It's too much effort to shield the room itself, but you
might want to try making the inverse square law work for
you by shielding all of the wireless antennae so that
the signal is too weak to travel more than a meter
or two. Put extra shielded wireless access points on
the conference tables so that everyone can place their
laptops within range of a signal.
But make sure that you thoroughly test the reception both
inside and outside the room to be certain that there are no
leaks.
No guarantees but I'd be interested to hear a report
if you try this.
--Michael Dillon