There is nothing in the constitution that requires the members of the Electoral College to vote for anyone.
When you voted for President you did not actually vote for Obama or McCain. Voters cast ballots for members of the Electoral college to meet and select a president.
When the electoral college was created communications was not very good. There was no way that voters in states could know about people who wanted to be president. So the founders decided that we would elect local people we knew and trusted to be members of the electoral college. They would meet to do the process of actually selecting our president.
They can vote for any qualified person they chose to vote for.
Aren’t the electors controlled by their states in some way? I know there are different policies - that is, in some states they are pledged to vote for particular candidates (as the voters have decided), in others they are not. Of course, if Obama were disqualified, I guess this would be a moot point anyway.