The Electoral College when they awarded him the 365 electoral votes he won in the November election and the Senate when they certified the results.
But I was speaking of the hypothetical case where a person was elected and took office, but then was found to not be eligible.
In the case of the presidency, that person is impeached and removed from office, and the vice-president becomes president.
Impeached on what grounds, the choices are limited to:
Treason, Bribery, or other high Crimes and Misdemeanors
Which one of those is "not eligible"?