https://www.washingtonpost.com/politics/2019/12/06/can-former-presidents-be-impeached/

Yes, you actually can impeach a former President.

Why? Article II, Section 4 of the Constitution, says, “The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors.” Article I, Section 3, says, “Judgment in cases of impeachment shall not extend further than to removal from office, and disqualification to hold and enjoy any office of honor, trust or profit under the United States.”