Without the safety valve of legal visas, making it harder to cross the border has mostly succeeded in encouraging people to stay (rather than cross back and forth) once they make it across the Southwest border.
The closest thing to a real success, a two-drug combination known as fen-phen, helped people lose as much as 30 pounds a year by making them feel satiated, but it caused heart valve problems in some and pulmonary hypertension in others.