Which is the only amendment that was used to repeal a previously ratified amendment?

You are probably referring to the 21st Amendment, which ended Prohibition. The 18th amendment to the Constitution (1919) made it illegal to manufacture, sell or distribute Alcoholic Beverages. It was supposed to reduce Alcoholism and lower crime rates (many crimes were related to drunkenness), but it turned out to be a very unpopular law, as well as a law that was nearly impossible to enforce. By 1933, the 21st Amendment had repealed it.