The framers of the Constitution were most concerned about government censorship of political opposition. But if the First Amendment guarantees the right to free speech, then it also prohibits any action — not just by the government, but also private groups and individuals — that aims to censor or silence unpopular viewpoints.
Here's an example. A Chinese dignitary is invited to a college campus to give speech. Midway through her remarks, a student activist begins to boo and yell about China's human rights abuses. The protester is so loud that he drowns out the dignitary's speech entirely.
This is called a heckler's veto, when the opinion of one angry person — or a group of people — tries to silence all debate. The term arose from a series of Supreme Court cases in the late 1940s. In each case, the police detained a public speaker out of fear that his speech would provoke a violent reaction from the crowd [source: Leanza]. The court sided with the speaker, arguing that it's the police's duty to protect free speech even if it incites anger in others. The "heckler" — whether it's a single protester or an angry mob — does not have the right to "veto" opposing viewpoints.
"A function of free speech under our system of government is to invite dispute," wrote the justices [source: Leanza].