I don't think you can actually switch cameras during a render session.  If you wish to abruptly change views you can reposition the camera in question at the proper keyframe and, using the animation palette, use the Constant keyframe type (grey bar).

Keyframes can be turned off for any object in the scene using it's properties tab.  Uncheck the Animating box.

I'd look for some tutorials on how to use the Animation Palette.  I bet the good Dr. Geep has some written up.  Using proper keyframes can add a lot to the Poser experience.  I use Constant when making pose sets.  Each frame can contain a pose and then those poses can be saved out to the library ...

Seriously, learn the difference between Spline, Linear and Constant keyframes, and learn how to use the Spline Break to place breaks in a sequence of actions.

Best Wishes,
Rªnce