NPC spawns on planets are set in the playfield files. There's 2 parts to this.

Biome spawns: NPCs can be configured to spawn in different biomes. There's no way to stop these from respawning.

Spawn zones: Spawn zones can be used to spawn NPCs near certain resource deposits or POIs. For hostile POIs these are usually deactivated when you destroy the POI's core. For example the Zirax patrols near their POIs will not spawn after you've destroyed the nearby POIs.

Both settings include respawn options. For most spawn zones they are set to never respawn so you can clear out the patrols near a POI without having to destroy the POI first, but for biomes they do respawn usually every one to three game days otherwise you'd run out of animals to kill for food.