(a) In the sense of to let, dejar is used.
Leave the poor child alone! He's not bothering you.
¡Déjale al pobre niño! No te molesta a tí.
(b) To leave off in the sense of to stop is rendered dejar de or cesar de, followed by the infinitive.
He left off coming when he found out that he was not congenial to us.
Dejó de venir cuando supo que no nos era simpático.
—BENAVENTE.
(c) To have something left is translated quedar or sobrar.
I know you have no time left for social life.
Ya sé que no le sobra tiempo para la vida social.—PÉREZ GALDÓS.