Are the coaches giving the gymnasts tools to handle their frustration? Are they just asking them to hide their feelings or are they saying that having frustration, fear, anger, self doubt or disappointment is right and normal and we all have those feelings, but that we should learn to control our actions when we feel that way?
Asking gymnasts to not roll their eyes when given a correction or to not kick or hit mats or equipment or throw things when they fail or watch the coach in the eyes and answer questions even when they are frustrated is RIGHT. It's teaching the gymnasts that they have to respect other people and their environment no matter how bad they are feeling. Having feelings is completely normal, but feelings are "just" feeling and we can ALWAYS control the actions that we take.
BUT, I would have a problem whit telling gymnasts they are not allowed to cry when they are sad or frustrated. The coaches should teach the gymnasts acceptable coping mechanisms and how to handle the negative feelings in a constructive way. Are the coaches having talks with the gymnasts individually, giving them a safe opportunity to tell how they are feeling about X, Y or Z problem? Are the coaches teaching them how to calm themselves when mad, disappointed or frustrated? Are they helping the girls to name their feelings and how to face them?
If they are doing all that, and some gymnasts still show disrespect in their behavior, then I think the board thing is ok. But otherwise, no.