If you are looking for a way to
block cell phone signals, but dont really want to get in trouble, could you get away with installing brass or copper mesh enveloping the
hall? It would act as a faraday cage and
block all the signals coming in... Would that be illegal? You could claim that you had an issue with RF interference if people complain about it, but if its illegal I probably wouldnt do it. Also, find out if your walls are sand filled cincerblock. Seems to make it more difficult to get signal through that.