(1) Halting all immigration (legal and illegal) would freeze the US population at some number. And then the population would begin to decline.
(2) Paying people to give up their US citizenship and emigrate would cause more gradual decline in the population.
(3) Mandatory 5 year contraceptive implants for all females in the US who receive more than half their income from social services would cause more gradual decline in population.
For those who will object that this will cause social security to run out of money, simply increase the year of beginning eligibility for social security by 1/2 year every year until reaching age 70 as the age of eligibility. Average life expectancy in the USA including fatal injuries is about 75 years of age, while excluding fatal injuries it is about 77 years of age. Most people nowadays can work until they are 70, no problem. The vast majority of jobs today are not high physical strength and endurance jobs, unlike 50+ years ago. Heck, most factory jobs now are supervising the robots.