I understand. But if we don't use your device against Godzilla, what are we going to do?

Matt's spot on of course. When the President says "So I've told people that if you're interested in avoiding World War III, it seems like you ought to be interested in preventing them from have the knowledge necessary to make a nuclear weapon," he's playing a rhetorical game that goes something like this--

We will prevent Iran from deploying a nuclear weapon.

To...

We will prevent Iran from knowing how to make a nuclear weapon.

To...

We will prevent Iran from watching the movies "War Games," "The Day After," "Terminator 2" or any other films that contain depictions of nuclear weapons.

To...

We will prevent Iran.