When a newspaper alerts its readers, “Warning, Ali G in area pranking people”, as the Dallas Morning News did recently, it is a sure sign that the boy from Staines, Middlesex, has made it in middle America.

The funky joker with the malapropisms, gangling limbs and suggestive gestures, played by Sacha Baron Cohen, is almost as recognisable in the US as in Britain due to the cable success of Da Ali G Show. Yet the attention for the publicity-shy comedian is not entirely welcome, for it is becoming harder to find stooges for his faux-naif act.

Americans are also cottoning on to Baron Cohen’s second-string character: the gormless, beige-suited journalist Borat Sagdiyev, “sixth best-known” Kazakh in the world.

It did not take long for strap-hangers on the New York subway to realise that the looming figure with the oddly familiar yet indefinable central European accent, who was lunging at men and trying to kiss them, was our mutual friend.

Even politicians — saps when it comes to free airtime — are wising up. Borat wanted to interview Congressman Jack Kingston about America’s quaint customs but was rumbled by an aide who heard a reference to Kazakhstan.

As Baron Cohen crisscrosses the US for a Borat mockumentary due out next year, ostensibly he has never been more successful. Borat: The Movie is directed by Larry Charles, the co-writer of the legendary television comedy Seinfeld. Baron Cohen’s fiancée, Isla Fisher, is being feted for her performance in Wedding Crashers.

At that premiere, however, Baron Cohen was tetchy when a photographer tried to take his picture without permission. And the clearest sign that his act is derailing is that Borat’s dupes are fighting back. One family in particular was taken in recently but is not taking its humiliation lying down. Like the best spin doctors, it is getting in its retaliation first by speaking out before the encounter is screened.

A few weeks ago George Matthews Marshall IV, the elderly owner of a 150-year-old plantation home in Mississippi, was delighted to offer a distinguished journalist from Kazakhstan some Southern hospitality. He did not realise he had been selected to play the role of fall guy.

Borat came loaded with stereotypes. When a maid brought in refreshments, he said: “Ah, I see you still have slaves here.”

“I was so shocked, I didn’t know what to think,” says Marshall, 75. “I told him people here get paid for a living.”

Borat then fished for compliments for George W Bush. Marshall, a staunch Democrat, was too polite to rubbish the president to a foreigner and so found himself reluctantly agreeing that Bush certainly had strong opinions. Marshall was also invited to abuse Yankees, Jews and gays, but claims he did not rise to the bait. Then Borat asked if he could bring in a woman friend.

Marshall is too much the gentleman to say much, but his friend and fellow guest Ron Miller fills me in. He says the woman, who was black with peroxide blonde hair and was wearing a leopard-print halterneck and plastic high heels, groped and fondled Baron Cohen over dinner. She was clearly a prostitute or pretending to be one.

Convinced by now that Borat must be some kind of weirdo or reality TV prankster, Miller politely escorted a reluctant Baron Cohen out of the house. “He kept banging on the door, saying, ‘I don’t understand. I just want to give my friend something,’ but I knew what he wanted to give her,” says Miller. The cameras kept running.

Miller told his daughter about it. She said immediately: “It’s Ali G. You’ve been Ali-G’d.”

“I think the show is going to be very funny,” says Miller, “but while you’re undergoing it, you don’t know where it is going to end. It’s very painful. You feel used and violated.”

It is not the first time Borat has run into trouble. When Baron Cohen (an observant Jew) got some rednecks to sing a “traditional” Kazakh folksong, Throw the Jew Down the Well, Americans and Jewish groups were offended and the Kazakh embassy complained.

Baron Cohen is in the business of offending, ostensibly to prick prejudice and humbug. But it feels rather more personal when the trust of a 75-year-old has been broken.

“Satirising a politician has historically been considered fair game, but to besmirch a family and try to entice them into expressing prejudice and bigotry is something quite different,” says Marshall. “The worst thing is he came here with prejudices about us.” So it could be time to send Borat home.