To move an object out of a namespace, you'll have to rename it.
CODE
rename Poly:pSphere1 :pSphere1
This will move pSphere1 from the Poly namespace to the root namespace. Once you've moved all objects out of a namespace you can delete the namespace. If you have nested namespaces (e.g. A:B:) and want to remove A, you'll first have to empty out both A and B and then remove B, and finally, remove A.