Well, if it's a Full Body Morph you can't do it in the Parameters Panel, because Delete Morph is grayed out for some reason when it's in the Body actor instead of a body part (at least in Poser 8). But you can do it in the Hierarchy Panel. Make sure Show Parameters is checked so you can see the dials in Hierarchy, then select the parameter in question and hit the Delete or Backspace key. Just like deleting anything else in Hierarchy.