In the Parameter's Pallet, Choose Props > Ground

Then click Properties

Uncheck Visible.

You can also uncheck Visible in Ray tracing, Cast shadows & Visible in Camera

OR if you want the Ground to catch shadows (if you have a background image you'd like to place behind the scene in a graphic program or within poser, Click Materials across the top row (Pose Material Face Hair Cloth, etc) Click Props > Ground > Advanced and then scroll down the Poser material pane and check shadow catch only

;>

Andy!