I won't let a player in the game, at the table, if his shirt is untucked.
If there is a dead ball (foul shot, inbounding) and I have the opportunity to say something to a player, I'll tell them to tuck it in.
I have, on occasion come across that "one player" who likes to pull his shirt out intentionally. He, I have reminded, and then asked him to leave the game when he does it repetitively, and said something to his coach.