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A protest outside a talk by a controversial 9-11 “truther” in Boerum Hill on Wednesday night turned violent when a protestor spat at a venue employee and the worker tried to slug him in return.

Things went south when a female protestor claimed that a barista at Atlantic Avenue event space the Brooklyn Commons had called her “fat b—-” when evicting her from conspiracy theorist Christopher Bollyn’s talk on “the Israeli-Zionist and neo-conservative cabal” he believes is responsible for the Sept. 11 attacks — prompting a male rabble rouser to hawk a loogie at the worker, who then swung at the guy and wrestled him to the ground along with a beefy security guard the space called in for the divisive event.

“He walked out the door and one of the demonstrators spat in his face and he tackled the guy,” said venue owner Melissa Ennen, who defied earlier demands from residents and activists to cancel the event, arguing that Bollyn’s ideas would “spark discussion.”

The spitter ran off after the scuffle and police arrived on the scene within minutes, though they didn’t make any arrests.

Around 30 people turned out for Bollyn’s talk, and roughly the same number gathered on the sidewalk, holding signs reading “Nazi scum your time has come” and “Our oppression is not up for debate.”

Among those in attendance was Councilman Brad Lander (D–Cobble Hill), who said he was appalled to learn that the owner of the self-described “progressive” work space was giving a platform to a man who believes the Jews had orchestrated the attack on the Twin Towers to force the world into war.

“He’s anti-Semitic, he’s a 9-11 truther, it’s gross,” said Lander, who is Jewish. “To learn that the owner of this space, who held it up as an inclusive venue turns out to sympathize with those vile, crazy points of view is really depressing.”

But attendees claimed they didn’t hear anything anti-Semitic or far-fetched in Bollyn’s two-hour talk — just well-known facts about who was really behind 9-11.

“Fifteen years later, it’s a plain obvious fact that those buildings were brought down by explosives — there’s a group of people who brought them down and anyone who denies it is not informed,” said Manhattanite Eric Rassi. “[Bollyn] is someone who was involved in a legitimate investigation of an obvious crime against the country and the facts and conclusions he’s reached are what these people don’t want to hear.”

Ennen, who has herself spoken at a truther event in the past, also claimed Bollyn didn’t say anything derogatory about Jews and said she also doesn’t believe the government’s version of what brought the towers down.

The lefty organizations that work out of the Commons by day had denounced the talk in an open letter and one, the Brooklyn Institute for Social Research, pulled all of its classes from the venue. But Ennen said she wouldn’t apologize for letting people exercise their first amendment rights.

“I don’t know what I’d be apologizing for, I would not,” said Ennen, the creator of the now-defunct Brooklyn Bridge Magazine. “I can say that I’m very, very sorry that this has been a breach of a lot of friendships and people that say I betrayed their trust, but I strongly feel that I am not in the position to be a censor.”

One activist said the event had not only tarnished her opinion of the Commons, it made the whole of Kings County look terrible.

“We’d expect that kind of s— to be hosted in Staten lsland but we do not expect that in Brooklyn,” said a Rock resident who identified herself only as Jen due to fear of retaliation.