I very briefly looked into this when I was refurbishing a house[1], and didn't see anything that wasn't ludicrously expensive, a scam, or both. Is there even a practical way of doing this? We've actually got a wire mesh in most of the walls due to the building materials used, but you can still pick up a (fairly weak) signal outside, mobile phones work, and windows defeat it (connection possible at >100m with generic hardware if you have line of sight through a window).

[1] Just to reduce interference from external WiFi, mostly as a curiosity

Oh this is definitely already a thing among conspiracy theorists, I saw an episode[0] of Grand Designs New Zealand recently in which a couple paid thousands of dollars for paint and wire mesh to block electromagnetic radiation. I guess there's still room for improvement though, because in the end, you could still make a cellular call inside.

[0] s02e02

Or just, you know, not operating participating Amazon devices. Call me crazy.