If you select the object, or objects in the GECK then hit the 1 key, they will turn partially transparant and can no longer be selected or moved. Hitting 1 twice in a row instead of once, makes them entirely invisible, so do this if they are going to be in your way visibly as well. Hitting F5 forces the geck to re-render the cell, making all objects visible, selectable, and movable once again.