That is the real problem. Radio signals don't stop at property lines. If you jam all the cell phones in a theater, you take out several surrounding office buildings too, at least intermittently. A rule of thumb I've used in ham radio is that if a signal goes 1 mile reliably, it will go 10 miles a lot of the time and 100 miles some of the time. And similarly for any distance.
Now what might work is a device to detect that cell phones are in use and complain about them. Cell phones transmit an identifying signal periodically even when you're not talking on them, so that the towers will know where they are.