There are basically two approaches, the first is to change the type of enclosures that you are using, a lot of the new designs are concentrating on focusing the sound only where it's needed and avoiding too much 'spillage' this is the aproach to take for band reinforcement in a concert setting.
The second approach is similar in concept but different in application, the concept is basically to get the sound where it's needed without 'exciting' the room too much, if it's more like a theatre/play type of setting I have found that using several small enclosures in a 'delay stack' type of arrangement can get a speaker close to everyone so that the volume can be kept reasonably low while still allowing everyone to hear everything clearly.
The worst possible thing to do is to set up one huge stack and pile a lot of power into a reverberant space, all that power is just going to bounce around in there and sound like shit.
If the hall is empty you can improve the sound a little by dropping the volume of the PA to a more 'Intimate' level.
I'm assuming that you have no authority or cash to acoustically treat the rooms.