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No way to stop this. This is part of the game mechanics. You can uncheck the box to stop the game placing any sims into houses which helps with keeping your worlds filled just with your own made sims but the game will always generate sims. These generated sims are needed for NPC roles such as the mail man/woman and so on.
In live mode, pause your game. Options Menu (Top Right icon on screen)>Game Options>Game Play> Fill Empty Houses ...unchecked.
If you place sims and households into 'My households" including those NPC sims (game generated) ones you like or have altered in CAS then this sort of helps to keep the spawning down.
Culling and My Households explained (click)The game has a sims population cap, once this is reached sim are deleted from the game around 3am (I think it is from memory) so if this population threshold is made up of your own sims then the game spawns in less NPC sims. (This is a very rough explanation of how the population cap works!) So it is possible though manipulation of sims placed into households and choosing the "Maximum Sim Count" to keep NPC or game generated sims to a minimum.
To find the 'My Households' Tab: Options Menu>Manage Worlds Then click on the icon that looks like 'two sims and a pencil' and choose Manage Households. A pop up will show and this is the Manage Households menu where you can manage your sims and the game generated sims. The Maximum Sim Count can be chosen here at the bottom on the menu.
Manage Household Button - via the map screen
Manage Households - My Households tab
