Defining a property to have an EMPTY range?

Hi Folks,

It dawned on me today that there are times when it may be useful to
state in an RDF Schema document (or an OWL document) that 

   "the range of this property is EMPTY"

Example: I would like to be able to "tag" a Document with a security
classification, e.g.,

   <Document>
       <secret/>
       <content>
           ...
       </content>
   </Document>

Note the <secret/> property.  Its purpose it simply to "tag" this
Document as having a secret classification.

Thus, in an RDF Schema (or OWL document) I would like to state:

    <rdf:Property rdf:ID="secret">
        <rdfs:domain rdf:resource="#Document"/>
        <rdfs:range rdf:resource="EMPTY"/>
    </rdf:Property>

Obviously, this is not correct, as there is no EMPTY in RDF (or OWL).

Am I looking at this incorrectly?  How would you do this?  What are your
thoughts?  /Roger