Is there a way to make it so a custom race can’t use other race populations?

I want to create an AI race that hates all living creatures and will only use its own race.

Race Affinity will be the Riftborn and they will “build" additional populations.

It won't do anything for other populations that show up through other means (i.e. quests, population on systems you conquer/assimilate) but you should be able to yoink the effect for the Religious law that prevents pop growth on populations other than your own and put that on a custom Trait.

looking at the religious law the modifier is 

<Modifier TargetProperty="ShouldPopulationGrow"         Operation="Force"        Value="-1"      Path="../ClassEmpire/ClassColonizedStarSystem/ClassPopulationStarSystem,PopulationNonMajor" TooltipHidden="true"/>

and I think for a faction trait you'd just need to remove the "../" and put it in the trait's SimulationDescriptor, I did something similar to make a trait that locked happiness like the religious law.