>>510940875 (OP)You sort of don't.
The Spartans did everything they could to crack down as hard as they could, down to actual controlled genocide of their slave people, and what ended up happening was that they ended up the worst out of every people down to a parody by the times Romans observed them.
The Spartan citizens themselves couldn't help but fall in love with their slaves when economic inequality inevitably took place in their society. And then the slaves themselves, despite systemic culling, actually ended up buying their freedom and just leaving or buying land nearby. Sparta ended up being the most mixed people between slaves and slave masters.
In fact, the only constant is that people will mix and create new races. Do you ever stop and think about Japanese people, how they're a mix between some local native islander people and moon face asians and they're a unique thing everyone calls Japanese? They themselves don't care, they consider themselves one race.
The only way to get your genes to survive throughout the ages is to have kids. God and nature don't care about your "ideals" as to why you're not reproducing.