To admonish is to scold. If you want to show someone you're not happy with his behavior, admonish him. It sounds better than "scolding," and it's less painful than spanking.
Coming to English through Old French from the Latin admonere "to advise, remind," admonish is always used with an eye on improving someone's behavior. The exact meaning of this formal verb varies in intensity depending generally on who is being corrected. If a child or subordinate is being admonished, it means "scold" or "rebuke" whereas if someone admonishes a person with equal standing, warn or advise are closer synonyms.