You can create custom groups and put whatever critters you want in them - either custom critters or critters from existing factions.
So, you basically need to re-create the faction as a custom group, and just not include whatever critters you're trying to avoid.
And for a while things were cold,
They were scared down in their holes
The forest that once was green
Was colored black by those killing machines
Could not find answer to this, know its here somewhere, but how do u edit the standard groups, ie, council to remove some of the types available?