WASHINGTON—The Supreme Court on Thursday unanimously threw out government sanctions against networks that aired fleeting profanity and nudity, saying regulators didn't provide fair notice that they were taking a tougher stance.

The decision, which focused on process rather than substance, offered little clarity to broadcasters after nearly a decade of legal battles over expletives uttered on live television and brief displays of bare breasts and buttocks. The high court declined to say whether government punishments in such...