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President Obama's pledge to open the White House up to the public through online forums faces an irksome challenge: a plague of Internet "trolls" -- troublemakers who work to derail cyber-conversations through harassing and inflammatory posts.

The problem became immediately apparent last month when Obama held an online "town hall" forum on the economy and invited the public to post questions on the White House Web site.

Those questions, in turn, were voted on by users to determine which ones the president would answer.

Three and a half million people participated in the event, but the "trolls" had their way: Following a coordinated campaign by marijuana advocates to vote their topic to the top of the list, questions on the future of the U.S. dollar and the rising unemployment rate were superseded by questions about legalizing pot as an economic remedy.

The president himself had a good laugh about the volume of marijuana-related questions, saying, "I don't know what this says about the online audience -- we want to make sure that it was answered. The answer is, no, I don't think that is a good strategy to grow our economy."

But the die was cast. Through a perfectly legal "underground" campaign, a relatively insignificant question had risen to the top.

For the White House, the question was not so much how to answer it -- but what to do about it, and how to prevent it in the future.

Unlike privately run Web sites, whose managers are free to remove nettlesome material, the White House finds itself searching for a way to combat these disruptive users without infringing on their right to free speech and inciting cries of censorship.

In 1997, the Supreme Court ruled that freedom-of-speech laws apply to the Internet after the American Civil Liberties Union sued the government over a federal law that would have criminalized the online transmission of obscene or explicit content.

"The law is well established, which doesn't mean government actors aren't going to try to restrict speech on the Internet, and that has happened," said Aden Fine, an attorney with the ACLU. "Restricting speech on the Internet is something we all need to be concerned about."

With all eyes on its cyber-presence, the White House will have to tread carefully.

Government institutions have the right to remove obscenities or limit comments to a specific subject area, but they are limited in their ability to remove comments that are merely repetitive or disruptive, said John Morris, legal counsel to the Center for Democracy and Technology.

"If the comment is on topic, they won't be able to remove that without raising some constitutional issues," he said. "Say they did a forum on national security issues, and activists say Guantanamo needs to be closed next month and not next year, and they flood the forum.

"It's on topic and they're not violating obscenity rules," explained Morris. "The government would have a difficult time pulling it down."

Ever since the first Usenet group was created in 1979, Web administrators have been contending with "trolls."

In 1998 Judith Donath, a professor at the Massachusetts Institute of Technology, chronicled various cases, including one where a troll logged on to a brides' forum and assumed the persona of an upper-crust matron, admonishing other users for supposed breaches of etiquette such as using a laser printer for their invitations rather than an engraver.

"Responding to a troll is very tempting, especially since these posts are designed to incite," Donath wrote. "Yet this is where the troll can cause the most harm, by diverting the discussion off the newsgroup topic and into a heated argument.

"Trolling is a game about identity deception, albeit one that is played without the consent of most of the players."

But scrubbing trolls' comments from a site presents a whole new challenge. And considering the increasing role online discussion board and forums play in people's lives, many don't consider it a laughing matter.

While Obama's marijuana advocates wouldn't technically be considered trolls, who are defined by their lack of definitive positions and a simple desire for disorder, these special-interest groups do muddle the president's message and related discourse.

How the White House will handle those and other disturbances in the future poses a conundrum.

Methodologies to control online discussion boards and forums are wide-ranging -- including everything from filters that keep out spam and obscenities, to more advanced software that analyzes language for an overly negative or harassing tone that a Web administrator might choose to flag.

Many sites allow users to assess the value of other posts and determine their prominence in the forum, in effect creating an online democracy.

But even that can prove problematic, as evidenced by Obama's first online Q&A last month.

Daniel Ha, CEO and founder of the Web site management company Disqus, said he was generally impressed with the White House's online savvy, but that there was still work to be done.

"Content submitted by other people is really hard to -- I don't want to say control, but maintain and set a quality to," Ha said. "A lot of the issues they're going to have to address as they go along. [An online forum] is just a medium. It's the same as if they held a rally in the park and people started making noise. That problem is going to exist no matter where you do it."

Morris said he didn't see any legal issues with using software to manage the forum, as long as the software was "viewpoint neutral."

"If you have software that looks at repetitive and harassing comments, regardless of the content, that's something the government can do," he said.

A White House spokesman did not respond to specific questions about what online tools they were using, only saying, "we are continuing to explore ways to use the Internet to increase the American people's access to the government and to engage with them about the challenges facing our country."

"People were informed that this was a community-moderated system, and people should remember that even though they may not like the viewpoint behind someone's question, everyone has a right to their opinion."

No announcement has been made on the date of the next White House online forum.