IANAL, but I assume if it's written in his will or some other legally binding document, then the courts would have to listen to it. Otherwise, his estate could probably win a court case, but it would be hinged on the movie company illegally profiting off of his image or something and would probably be more difficult to stop. Like when they make ads with fake celebrity endorsements.
I suspect they just pay the estate lots of money for image rights. Which is grim, but a person cannot exert power over their likeness when they are dead. As always, capitalism pays for what it wants, without regards for morality.