China's coercive family planning of the past 20 years has slowed their increase to 10 million a year, and has prevented their current 1.26 billion from being 300 million more. Their goal is to peak at 1.6 billion around 2050. Before we start looking at coercive methods of reducing births, lets first stop demanding reproduction, and start allowing self-determination. Within the borders of freer countries like the United States, migration is determined mainly by economics and personal preference. Between countries, however, restrictions on migration are plentiful and often capricious.
Enforcement is always at gun point.
It has been suggested that increasing these restrictions will reduce population growth, and will therefore benefit the environment. However, looking at Earth from a broader perspective, say from space, reveals that all those lines arbitrarily dotted across the planet do not help the biosphere as a whole. If one of the methods we choose for preserving Earth's ecosystems is to draw lines on the ground and dare people to step over them, we should be drawing them around wilderness preserves, as many African nations are doing. Restrictions on in-migration are needed in the remaining vestiges of Nature, in the fragile ecosystems which can't absorb more human impact. The idea of restricting our own migration within our borders may not have the appeal that restricting in-migration by foreigners has, but for Earth's sake, that's the direction we should be migrating in.