The Electors, as I’m sure you know, don’t make a decision as to whether the candidate they’re pledged to is eligible, as the Founders no doubt intended. They are merely a formality by which the will of the voters, potentially somewhat distorted by the peculiarities of the winner-take-all system in most states, is expressed.
Since they no longer fill their original constitutional role, it defaults to the voters of each state.
Any state legislature, if it so chose, could return its own electors to what the Founders intended, choosing men who would exercise their own judgment on who to vote for. But that’s about as likely to happen as me being elected President.
The Constitution establishes no other role for the Electors than to vote for the president. Your suggestion that they are to determine eligibility is pure speculation and there is no historical precedent for the electors ever performing that function.
It is Congress who can stop an ineligible candidate by refusing to certify his/her Electors or count the Electoral votes from any particular state.
Today, 21 states allow electors to not necessarily vote for the winner of the state’s popular vote (”faithless electors”). Electors in those 21 states could eadily deny any candidate the necessary 270 electoral votes that are required.