Reddit, an online bulletin, said it has removed offensive forums that attacked gays, obese people and blacks in order to stem some of the harassment for which the site has long been criticized.
In an announcement posted online by company executives including Chief Executive Ellen Pao, Reddit said it was banning behavior, not ideas.
"We want as little involvement as possible in managing these interactions but will be involved when needed to protect privacy and free expression, and to prevent harassment," the company said.
The offending forums, known as subreddits, attracted thousands of subscribers.
The change has already been met with mixed reviews by commenters on the freewheeling site, which has also been called the front page of the Internet for aggregating some of the Web's most-viewed stories, posts and pictures. The crackdown drew criticism from some fans for conflicting with the site's stated support of free speech.
"Censorship, man. I thought there'd be more time before it really hit reddit," one person wrote.
"While we do not always agree with the content and views expressed on the site, we do protect the right of people to express their views and encourage actual conversations according to the rules of reddit," the San Francisco-based company said in its statement.