Disarm is also mostly ineffective, as the Enforcer or Crowd are always quick to toss down a new weapon, and there seems to be an entire Armoury's worth of weapons lying about the Arena floor which can be grabbed up at a moments notice (including an ample supply of Old Fists...) It's a miracle the gladiators don't trip, there's so much crap lying about...
Of the Signature Moves, only Sweep seems to have any real effect, and only then to trigger the "Opponent is On The Ground" battle order. I haven't seen any appreciable reduction in attack activity or damage level from a prone fighter