what trick does hades pull to make sure persephone is not leaving him for good?

It was not Hades that tricked Persephone. Zeus had decreed that Persephone could return to her mother if she had not yet eaten anything in the world of the dead. Persephone was overjoyed that she was going to be returned home. There was an evil gardener who was angry with Demeter for having turned him into a lizard many years before. He coaxed her into eating some pomegranate seeds. Because of this Persephone could not return to her mother. Zeus later vetoed this and let Persephone spend six months with mother and six months with Hades.