If you haven't got anything nice to say, then it's time to disparage someone. It means to belittle or degrade a person or idea.
Disparage is a specific way to describe a certain kind of insult, the kind that secures the insulter's place as superior. It often refers to an opinion or criticism lobbed in print or via word of mouth, not necessarily an act done to someone's face. If someone or something is being disparaged, you will often find a competing interest in the wings.