There is no provision for a NBC like X having his citizenship revoked. He can renounce his citizenship, but nobody can revoke it, no matter what the person does.
A person can “renounce” his citizenship by performing acts that, in essence, renouce it, even if that wasn’t his intent. For example, voting in an election in another country. Possibly flying on a foreign passport. Possibly getting student loans as a foreign student. Also, if born a dual citizen, not letting the non-US citizenship go away upon reaching the age of majority. That’s my understanding.