restricting namespace usage within a document?

Is there a way via XML Schemas (or some other validation method) to
restrict the usage of namespaces within a document? I am both interested
in restricting which namespaces can be used within a document as
children of a certain element, and restricting all uses of namespaces
except under certain elements. DTDs do not provide this: I was under the
impression that XML Schemas did, but I have been unable to find any
information on whether or not my assumption was correct.

(please CC me directly, I am not on the mailing list)

-David Waite