Who destroyed the Spartans?

The Spartans were not destroyed by anyone in particular. Their big problem was that, as a city-state based on an exclusive minority which was peridoically engaged in warfare, that warrior mnority was a shrinking asset. Whenever they suffered a serious defeat, the downward move in population was accelerated. Their loss to the Theban alliance in the early 4th Century BCE was a critical point from which they never fully recovered. Although they made an attempt at recovery in the late 3rd Century, it was temporary, and Sparta ceased to be a power in the Hellenic world. Thereafter they faded out, being recognised for their past achievements, but counting for nothing in Hellenistic affairs.