Sims 4 - how to trap a sim???

[Lazygoku]

Whenever I trap a sim behind walls, they teleport out when they feel like leaving. How can I trap them without having them in my household?

[Playalot]

Welcome to the forum. 
If you stay on the lot where you have trapped them or locked them in a room... they will not be able to get out. The game mechanics actually create 'new' sims every time your sims travel. This is why when you leave your lot and return home your sims will often be doing different actions or will be waiting to give the returning sim a hug. Same data, newly 'created'  bunch of pixels.

Short answer... lock the door and don't leave the lot.

[Magpie2012]

@Playalot I think the actual question is about trapping NPC sims or uncontrolled sims. As far as I know, they teleport out regardless as they are not part of the household. Just out of curiosity (so glad I'm not a cat!) Why do you want to trap a random townie?

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[Playalot]

Yeah, I know but I have trapped non-household members by doing this... just lock the door. I haven't had one teleport out yet. They say 'goodbye' and you can't interact with them for a bit but they stay in the locked room.
If you wall them in the game will eventually teleport them out as it will 'assume' they are stuck. Just like when one of your own sims gets stuck coming through a door... eventually the game teleports them away from the door and out of being stuck.