No es justo...as an idiom....
Imagine you are 3 years old and you want to say "it's not fair".... Would you say 'no es justo' or something else? Or just scream and scream and scream....
2 Answers
"No es justo" is correct.
¡No hay derecho! would be my first thought or ¡Es injusto! Anyway I am not a native so better wait for other answers.