When the government blocks the publication of material it does not want released this is known as?

Prior restraint is a legal term related to censorship in the United States referring to government actions that prevent communications from reaching the public. Its main use is to keep materials from being published. (from en.wikipedia.org/wiki/Prior_restraint)

"Classified" material. In the United States, the normal levels of classification are Confidential, Secret and Top Secret.