Is there a defense against Cell Phone Jammers «

So I was hanging out with some peeps last week and I came across this cell phone jammer. I’m pretty sure they are not legal here, but I think that it’s very interesting that for about $30 you can get a device that jams up 3g, CDMA, and GSM, all in one box. This particular board runs off a cellphone battery, and works up to about 15 feet. It was acquired in China.

You could always just go here and get your own industrial cell phone jammer.

I don’t know much about cell jamming, but I know they use them around prisons, during riots, and other sensitive situations. Is there any defense against these? What can we do to provide uptime with these devices accessible? What if someone had a more powerful one (say 500 or more feet) and they setup shop outside your business?