The Beginning of the Ending

A US federal law, the "Act Prohibiting Importation of Slaves" went into effect on January 1, 1808. This legally ended the transatlantic slave trade. It was December 1805 when Senator Stephen Roe Bradley from Vermont, brought out a bill against the importation of certain persons therein described [the word slaves was added latter] into any port or place with in the United States after the first day of January 1808. President Thomas Jefferson signed the bill into law on March 3rd 1807. The fifteenth Congress changed the law, calling the act of importing slaves piracy, and said that any citizen of the US found guilty of such piracy could be given the death penalty. It would of course take another fifty-seven years and a war to end slavery.

Other reading
Speech of C.M. Clay of Fayette, in the House of Representatives of Kentucky: upon the bill to repeal the law of 1835, "to prohibit the importation of slaves into this state".