8 U.S. Code Title 8 Section 1182 - Inadmissible Aliens:
a) Classes of aliens ineligible for visas or admissionExcept as otherwise provided in this chapter, aliens who are inadmissible under the following paragraphs are ineligible to receive visas and ineligible to be admitted to the United States...
See Cornell University Legal Information Institute for the full text of 8 U.S. Code Title. 8 Section 1182 - Inadmissible Aliens.