Another thing you could do is use imdb as a resource. Say something like, if an actor is listed as one of the first 6 cast members any other film where he is listed in the first 6 cast members is now unavailable.
So looking for example at Oceans 12
http://imdb.com/title/tt0349903/
This would eliminate Pitt, Zeta-Jones, Clooney, Kross, Roberts, and Tiffany but people could still pick Crash since Don Cheadle wasn't listed in the top 6 for Ocean's 12