Go to SP>General Options>Lot Options:Immigration/Emigration>Immigration Gauge>type in 1000. That will have to be the pressure for the town to populate. Then you can populate the world at your whim with the sims that you want. Note that this will not stop service sims from generating. If you have stores and other services, you will have to remove them to stop ALL populating but it will stop resident sims from being game-generated.
How do I know this? Well, I'm playing an apocalypse fic, which basically meant that I needed to DEPOPULATE the town in a hurry. Think nuclear apocalypse.
