“She was using the word ‘like’ all the time. She sounded like a Valley Girl,” said the architect, Andrew, who declined to give his last name because what he did next was illegal.

Andrew reached into his shirt pocket and pushed a button on a black device the size of a cigarette pack. It sent out a powerful radio signal that cut off the chatterer’s cellphone transmission — and any others in a 30-foot radius.

“She kept talking into her phone for about 30 seconds before she realized there was no one listening on the other end,” he said. His reaction when he first discovered he could wield such power? “Oh, holy moly! Deliverance.”

As cellphone use has skyrocketed, making it hard to avoid hearing half a conversation in many public places, a small but growing band of rebels is turning to a blunt countermeasure: the cellphone jammer, a gadget that renders nearby mobile devices impotent.

The technology is not new, but overseas exporters of jammers say demand is rising and they are sending hundreds of them a month into the United States — prompting scrutiny from federal regulators and new concern last week from the cellphone industry. The buyers include owners of cafes and hair salons, hoteliers, public speakers, theater operators, bus drivers and, increasingly, commuters on public transportation.

The development is creating a battle for control of the airspace within earshot. And the damage is collateral. Insensitive talkers impose their racket on the defenseless, while jammers punish not just the offender, but also more discreet chatterers.

“If anything characterizes the 21st century, it’s our inability to restrain ourselves for the benefit of other people,” said James Katz, director of the Center for Mobile Communication Studies at Rutgers University. “The cellphone talker thinks his rights go above that of people around him, and the jammer thinks his are the more important rights.”

The jamming technology works by sending out a radio signal so powerful that phones are overwhelmed and cannot communicate with cell towers. The range varies from several feet to several yards, and the devices cost from $50 to several hundred dollars. Larger models can be left on to create a no-call zone.

Using the jammers is illegal in the United States. The radio frequencies used by cellphone carriers are protected, just like those used by television and radio broadcasters.

The Federal Communication Commission says people who use cellphone jammers could be fined up to $11,000 for a first offense. Its enforcement bureau has prosecuted a handful of American companies for distributing the gadgets — and it also pursues their users.

Investigators from the F.C.C. and Verizon Wireless visited an upscale restaurant in Maryland over the last year, the restaurant owner said. The owner, who declined to be named, said he bought a powerful jammer for $1,000 because he was tired of his employees focusing on their phones rather than customers.

“I told them: put away your phones, put away your phones, put away your phones,” he said. They ignored him.

The owner said the F.C.C. investigator hung around for a week, using special equipment designed to detect jammers. But the owner had turned his off.

The Verizon investigator was similarly unsuccessful. “He went to everyone in town and gave them his number and said if they were having trouble, they should call him right away,” the owner said. He said he has since stopped using the jammer.

Of course, it would be harder to detect the use of smaller battery-operated jammers like those used by disgruntled commuters.

An F.C.C. spokesman, Clyde Ensslin, declined to comment on the issue or the case in Maryland.

Cellphone carriers pay tens of billions of dollars to lease frequencies from the government with an understanding that others will not interfere with their signals. And there are other costs on top of that. Verizon Wireless, for example, spends $6.5 billion a year to build and maintain its network.

“It’s counterintuitive that when the demand is clear and strong from wireless consumers for improved cell coverage, that these kinds of devices are finding a market,” said Jeffrey Nelson, a Verizon spokesman. The carriers also raise a public safety issue: jammers could be used by criminals to stop people from communicating in an emergency.

In evidence of the intensifying debate over the devices, CTIA, the main cellular phone industry association, asked the F.C.C. on Friday to maintain the illegality of jamming and to continue to pursue violators. It said the move was a response to requests by two companies for permission to use jammers in specific situations, like in jails.

Individuals using jammers express some guilt about their sabotage, but some clearly have a prankster side, along with some mean-spirited cellphone schadenfreude. “Just watching those dumb teens at the mall get their calls dropped is worth it. Can you hear me now? NO! Good,” the purchaser of a jammer wrote last month in a review on a Web site called DealExtreme.

Gary, a therapist in Ohio who also declined to give his last name, citing the illegality of the devices, says jamming is necessary to do his job effectively. He runs group therapy sessions for sufferers of eating disorders. In one session, a woman’s confession was rudely interrupted.

“She was talking about sexual abuse,” Gary said. “Someone’s cellphone went off and they carried on a conversation.”

“There’s no etiquette,” he said. “It’s a pandemic.”

Gary said phone calls interrupted therapy all the time, despite a no-phones policy. Four months ago, he paid $200 for a jammer, which he placed surreptitiously on one side of the room. He tells patients that if they are expecting an emergency call, they should give out the front desk’s number. He has not told them about the jammer.

Gary bought his jammer from a Web site based in London called PhoneJammer.com. Victor McCormack, the site’s operator, says he ships roughly 400 jammers a month into the United States, up from 300 a year ago. Orders for holiday gifts, he said, have exceeded 2,000.

Kumaar Thakkar, who lives in Mumbai, India, and sells jammers online, said he exported 20 a month to the United States, twice as many as a year ago. Clients, he said, include owners of cafes and hair salons, and a New York school bus driver named Dan.

“The kids think they are sneaky by hiding low in the seats and using their phones,” Dan wrote in an e-mail message to Mr. Thakkar thanking him for selling the jammer. “Now the kids can’t figure out why their phones don’t work, but can’t ask because they will get in trouble! It’s fun to watch them try to get a signal.”

Andrew, the San Francisco-area architect, said using his jammer was initially fun, and then became a practical way to get some quiet on the train. Now he uses it more judiciously.

“At this point, just knowing I have the power to cut somebody off is satisfaction enough,” he said.