[Erasmus]

Passive interference, i.e., shielding, is legal. But it's expensive and tricky to do right.

Expensive, to say the very least!

And, since nobody's mentioned it yet, jamming an area the size of a theater without also shielding it will block significant areas outside the theater as well.

[Shalom Israel]

Expensive, to say the very least!

I should comment, though, that the market will provide. A japanese fellow has already invented wood panelling with embedded nickel-cadmium filings that blocks cell signals. His ultimate goal is to market the panelling through Home Depot.

And, since nobody's mentioned it yet, jamming an area the size of a theater without also shielding it will block significant areas outside the theater as well.

Not necessarily. The secret is multiple jamming bubbles of controlled radius. A similar trick was used when I was at Symbol to provide WiFi in an office that couldn't be sniffed outside the building.