I've seen mouth guards hanging out in the last season or two as Curry's profile has risen. That being said, I'll usually warn a player to the tune of "Either keep that thing in your mouth or get rid of it". I've not had to deal with a player continuing to let it (mouthpiece) hang out (yet). I think I'd choose the route of sending him/her out of the game before whacking them.
My line of thinking is that the mouthguard is likely covered with body fluid...potentially transferring it to other players is just as dangerous as an open cut or blood on a uniform/court.
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Calling it both ways...since 1999
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