Free Speech, Freedom from Government Retaliation

Lessig says in code that the right to free speech protects citizens from retaliation by the government for speaking one’s mind. It is imaginable that this amendment would rightly protect those who sought reform in an inefficient (or perhaps even corrupt) government. A government afraid of change could abuse its sovereign power by imprisoning those who spoke out against it. This practice is common amongst authoritarian forms of governments. Free speech is not compatible with dictatorships for example and is therefore not permissible in countries under those conditions. Dictators like Hitler and Pol Pot would throw anyone who were against them in jail or execute them because if they were to gain broad support they could possibly pose a problem. The way these people would gain support would be by communicating their ideas, this is what speech is. In the case of the United States, our society vehemently protects the right to free speech because the opposition to it is reminiscent of these authoritarian regimes. Yet, free speech thrives in the US and our government is not endangered by it, but could some speech be harmful to our government and our society as a whole? Look at conspiracy theories and the people that believe them. These ideas develop a following of people that basically become skeptical of the government. This is allowed by freedom of speech. Would it be possible for the perfect conspiracy theory to be developed that could convince enough people to overthrow the government. What limits need to be placed on free speech that would prevent the perfect liar from creating a following?