They can completely DQ Obama and not have him up for vote at all with no alternative but to vote for McCain / Palin. If it ultimately goes to The House, they could only have the option to vote for McCain, McCain or McCain.
http://www.rallycongress.com/constitutional-qualification/1244
Unless some elector, or a group of them votes for someone other than Obama. As Congressmen Billy Bob (John Armor) indicates, if he were DQ'd before the electors vote they would *all* be free to vote for someone else.
Of course he also points out that in most states (but not all???) a vote for someone else *before* Obama was DQ'd would not be allowed, and would be an "implied resignation". The states determine the rules for electors, within some very broad Constitutional requirements, so "they can do that".