An all-purpose phrase, as long as the purpose is to stop Richard talking, responding to an insult, kicking a ball, sitting near you, or otherwise having any dealings with the world around him.
Best said in an exhausted and really loud voice, so everyone turns around to see what it was that Richard was trying this time.
After some months of such training, Richard will automatically not try it, and require only a raised finger and eyebrow to shut him up, if a look of hopeful interaction flickers across his face, or any sounds start to come out of his mouth.