They did her a great/terrible/grave injustice by not allowing her to file a complaint.他们不允许她抗议，这对她极其不公平。

I think you do the book an injustice when you call it “trash.”我认为你称这本书为“垃圾”是不公平的。

The weak punishment does an injustice to the criminal's victims.轻罚罪犯是对受害者的不公。