The law banning importation of slaves after 1808 was enforced by the United States Navy. The Navy kept a squadron of war ships off the West African coast from 1819 to 1861 specifically to prevent slave ships from delivering their cargos to the new world. In the 1840s a second squadron was added to patrol off the coast of Brazil. In the 1850s a squadron started anti slave trade patrols in the Caribbean.
The Navy’s success was not great, only about 100 slave ships were seized during the 42 years active anti-slave patrolling. By comparison, the Royal Navy captured over 1600 slave ships. Of course the Royal Navy was considerably larger than our Navy during those years.