Cell phone jammer
The wave bubble project has a lot of schematics and ideas:http://www.ladyada.net/make/wavebubble/Not sure about adjustable range, but I'm sure you could disturb/limit the transmission power if need be. Ask a HAM geek if you're not one yourself. Keep in mind that cell jammers are illegal in tons of places, in many even to possess the device without using it.
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or just buy a yapper zapper. google it. phone jammer