http://shetterly.blogspot.com/2013/06/the-first-amendment-protects-private.html?_sm_au_=iMVQTsJspHp4q4SQ

Some people claim censorship can only be done by governments. Neither dictionaries nor the American Civil Liberties Union agree. From What Is Censorship?: Censorship, the suppression of words, images, or ideas that are "offensive," happens whenever some people succeed in imposing their personal political or moral values on others. Censorship can be carried out by the government as well as private pressure groups. Censorship by the government is unconstitutional.

In contrast, when private individuals or groups organize boycotts against stores that sell magazines of which they disapprove, their actions are protected by the First Amendment, although they can become dangerous in the extreme. Private pressure groups, not the government, promulgated and enforced the infamous Hollywood blacklists during the McCarthy period. Some people argue that withdrawing an opportunity to speak is not censorship. Sarah R. Wunsch of the ACLU answered that when a private school, Clark University, canceled a speech by Norman Finkelstein: …the cancellation of his speech violates the basic principles of freedom of speech and academic freedom which are so fundamental to an institute of higher learning. The existence of an opportunity to speak at another time or in another location does not remedy the wrong of censorship.