United States v. O'Brien (1968)
In March 1966, during the Vietnam War, David Paul O'Brien and three other men burned their draft cards on the steps of a courthouse in Boston .  O'Brien was arrested by an FBI agent and charged and convicted under a section of the Selective Service Act that made it a crime to "knowingly destroy" a draft card.  The Supreme Court upheld his conviction on the grounds that preventing the destruction of draft cards furthered an important government interest that justified a restriction of the First Amendment. 

http://www.oyez.org/cases/1960-1969/1967/1967_232/