There were several US spies that got away with it, including Ted Hall and the Cohens.
PBS (surprisingly) ran an excellent program a few years ago, “The Red Files.” Click the red links at the bottom of this page:
http://www.pbs.org/redfiles/kgb/index.htm
The U.S. let Ted Hall get away with it, because they didn’tt want the Soviets to know about the Venona project.