My church sits about 2500, normally we had drapes on stage and behind stage to help absorb the sound but then the ceilings were not in place, I gave specifications to my boss to use perforated boards with acoustic materials in them but the building engineer did otherwise. My hall I'd filled with reverbs. I need to pad the walls. What method of padding would you advice?
You can introduce as many bass traps etc. as possible. Simple wall surface treatment doesn't do much.
You probably won't be allowed to do this, as the visual aspect of the room will be considerably affected. So maybe what you really need is an amplification strategy. More speakers, closer to the audience, run quieter.
Maybe the PA system is already installed. In that case all you can do is keep levels as low as possible. Forget about 'rock band PA' just settle for 'audibility and clarity'.