That's Gotta Hurt

I'm thinking about today's L.A. Times story about the proliferation of cell phones in state prisons, this line in particular:
Some of the cellphones are smuggled in pieces, secreted in packages, books and body cavities. Multiple people can use the same phone.
On a less scatological note, isn't this problem easily fixed? Doesn't technology exist to block out cell-phone frequencies? Why not put that technology to use in our prisons?