Best answer is probably just to use Facebook Connect- they're already pouring money into CS reps whose job it is to take down fake accounts. Blocks out people who aren't on Facebook, but it's a simple solution to something that can otherwise be a huge timesink.
How much of CNN's viewer base is Facebook-connected? The point of this sort of polling is to drive traffic - this ain't gonna happen if most of the people can't vote without going through a big signup for a service they don't want.
