If the US Govt invalidated your passport, what does it mean?

US government is implementing a ban on travel to North Korea, with the penalty being that anyone travelling there on US passport will "have their passport invalidated". (on mobile can't link to news story)

What does that mean. If someone defied the rule and went, would they not be allowed back in US? But without any passport they'll just be stuck in an immigration holding area? What exactly does it mean to have your only US passport invalidated?