You can delete FBMs and PBMs in the Hierarchy Editor, after clicking the "Show All Parameters" box. You delete them just like props and figures. Unfortunately the associated PBMs don't go away along with the parent FBM. You have to delete those separately.

I agree that Poser should not make deleting FBMs impossible from the Parameters Panel (the Delete command is grayed out).  You can delete PBMs there, why not FBMs?

I endorse Phil's PocketKnife. It's got a lot of cool python functions in addition to deleting morphs.