You have two choices. You can just delete the ground plane. This will remove the ground to cast shadows onto. Just click on plane 1 in the wireframe vew and press delete. The problem with this is your letters will still cast shadows on your other letters. To set the letters so that they won't cast shadows click on the M next to the selected wireframe object. Click on the circled triangle when thet material window pops up. Uncheck the Cast, receive and self shadows on the menu.