Legally, it would take a Consitutional amendment before hand, such as this:
1- The President of the United States of America together with the Vice President shall be elected to the same four year term on the day established by the Congress by the electorate of the various states. The electors in each state shall have the qualifications requsite for electors of the most numerous branch of state legislatures.
2- In the event that no Presidential or Vice Presidential candidate recieve a majority of the votes cast, a second election will be held on a day established by Congress, and within four weeks of first election, in which the electorate of the various states shall vote again, this time chosing between the candidates who received the first and saecond most votes in the first election. The winners of this election shall serve as President and Vice President of the United States for the upcoming term.
I suppose in theory, had the problems W/butterfly ballots, hanging chads and absentee ballots been a lot worse (God forbid), Florida's Secretary of State, Kathleen Harris could have refused to validate the results. That though wouldn't get you the runoff election you seek. It would only the Florida legislature selecting its own slate of electors and a mess for Congress to sort out in January when it received the electoral college vote results.
For a more recent example, look at last November's Minnesota US Senate race. The top 2 vote getters each received about 42% of the vote. It's a shining example of why there should be run off elections if no candidate wins a majority. Why hasn't that happened? Because there's no provision for it in the Minnesota Constitution. There is presently no provision in the US Constitution permitting a runoff election for the P/VP OTUS. No one and no body (judicial, legislative or executive) has the power to authorize/order a runoff presidential election.