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Scott Rubin, a spokesman for YouTube, said the site takes any posts “inciting others to commit violent acts … very seriously,” with a permanent ban being a potential consequence. YouTube will hand over to authorities any content that may violate the site’s guidelines.

The site also has a safety tool allowing users to flag “threatening comments” and round-the-clock staffers monitor and sometimes report those submissions to authorities.

Users can vote comments up or down and block offensive or objectionable language.

The White House has an open comments policy on its YouTube page, where user cherrybombz454 complained about Mr. Obama’s policy toward Iran.

“Worst President Ever!” the user wrote.

Plenty of other Facebook polls are judging Mr. Obama, asking whether he’s a “socialist,” and more than 34,000 have voted in a poll asking whether Mr. Obama is “destroying our country.” But most users said the assassination poll crossed the line.

“You do not make jokes about killing leaders you disagree with,” Keith John Sampson of Indianapolis scolded Facebook users.

He said when he attended a protest of President George W. Bush in May 2003, he reported to Secret Service a threat another protester made against the president.

“I could not take the chance that the fool at the demonstration might not decide to take action on his own,” he said. “No one detested President Bush more than me … [b]ut I never wished him harm.”

Mr. Farmer turned over to the Secret Service the poll creator’s numerical identifier but still the internet was flooded with comments from critics who thought he was responsible for its content. He said he was relieved that the tech-savvy authorities understood that wasn’t the case.

Mr. Farmer admitted he failed by not noticing the out-of-bounds poll until it had already become a news story.

“Now it’s back to the drawing board and making it better,” he said.