I have a Toro ZTR with a much smaller Kohler engine, and my dealer also told me to shut it off at full throttle to prevent the backfire. My owner's manual says to move the throttle to "between Fast and half throttle" and turn off the key. Actually, that only works part of the time. But I finally found the way to shut if off without a backfire EVERY TIME! When I are ready to kill the engine, I pull the throttle back to idle, then shove it forward to full throttle and immediately turn off the key as the engine is spooling up. Under those conditions, it NEVER backfires.
No guarantee it'll work on your machine, but try it, I think you'll like it.