I had this same problem with a job we shot via shotgun in a large, live ballroom. One of my guys suggested trying the Cedar DNS1000. We rented it from Bexel for about $100/day and it worked fabulously! I even tried it on some music and it killed ALL of the reverb. The only trick with it is that you need to run live sound into/out of it, it's not a plug-in... and it requires digital in/out, not analog. But we just borrowed a digital mixing board to do A/D & D/A conversion and it went great. Good luck!
Troy Witt, President
Take One Productions, Inc.
http://www.TakeOneDigital.com video production & webcasting