What would Sparta do to those they conquered?

In most of the cases they would be enslaved.

Explanation:

When the Dorians came on the soil of Peloponnese they encountered natives who were conquered by them. Most of those people were enslaved and were considered as state slaves (helots). The other part was not enslaved, but was left to leave on the margins of cities were they were engaged in agriculture and trade. None of them had any political rights in Sparta.