I coped this from some site because I'm lazy. This also presumes you mean the US law regarding free speech, because there is no law about free speech for forums.

"Freedom" does not mean "free of restrictions." On several occasions the U.S. Supreme Court has noted that there are no "absolute" freedoms: "Conduct remains subject to regulation for the protection of society. The freedom to act must have appropriate definition to preserve the enforcement of that protection."(1 ) Therefore, although the First Amendment to the U.S. Constitution provides that "Congress shall make no law . . . abridging the freedom of speech," the U.S. Supreme Court has recognized that the government may do just that: "[T]he unconditional phrasing of the First Amendment was not intended to protect every utterance."(2)

Some types of speech that the U.S. Supreme Court has ruled are not subject to First Amendment protection include:

* Obscenity: "If a state obscenity law is [appropriately] limited, First Amendment values are adequately protected by ultimate independent appellate review of constitutional claims when necessary." Miller v. California, 413 U.S. 15 (1973).
* Defamation: "[I]t is recognized that punishment for the abuse of the liberty accorded to the press is essential to the protection of the public, and that the common-law rules that subject the libeler to responsibility for the public offense, as well as for the private injury, are not abolished by the protection extended in our Constitutions." Near v. State Of Minnesota Ex Rel. Olson, 283 U.S. 697 (1931).
* Causing panic: "The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic." Schenck v. United States, 249 U.S. 47 (1919).
* Fighting words: The U.S. Supreme Court held that the First Amendment does not protect "fighting words -- those which by their very utterance inflict injury or tend to incite an immediate breach of the peace." Chaplinsky v. New Hampshire, 315 U.S. 568 (1942). However, in Brandenburg v. Ohio, 395 U.S. 444 (1969), the Supreme Court held that the First Amendment protects speech that encourages others to commit violence, unless the speech is capable of "producing imminent lawless action. Even advocating "illegal action at some indefinite future time" is protected. Hess v. Indiana, 414 U.S. 105 (1973).
* Sedition: "We reject any principle of governmental helplessness in the face of preparation for revolution, which principle, carried to its logical conclusion, must lead to anarchy. No one could conceive that it is not within the power of Congress to prohibit acts intended to overthrow the Government by force and violence." Dennis v. United States, 341 U.S. 494 (1951), where one of the "acts" which the Court found the government could prohibit was to knowingly and willfully "advocate and teach the duty and necessity of overthrowing and destroying the Government of the United States by force and violence."