I didn't go into it in detail, but it isn't even that I got the NPCs to start babbling about Taylor Swift. What is was was just that they knew that she was a musician, and as such, might be at the tavern. That's very hard to remove.
One concrete example I'm sure these Skyrim mods aren't using is: enums in structured outputs [1] with a finite list of locations/characters/topics/etc that are allowed to be discussed. The AI is not allowed to respond with anything that is not in the enum. So you can give it a list of all the locations in the game in a huge array and it would be forced to pick one.
[1] https://platform.openai.com/docs/guides/structured-outputs#a...