<FIELD name="Iteration Path - Validation" refname="MyFields.IterationPathValidation" type="String">
<HELPTEXT>Hidden field used to validate Iteration Path</HELPTEXT>
<WHEN field="System.IterationId" value="XX">
<COPY from="value" value="Restricted" />
</WHEN>
<PROHIBITEDVALUES not = "[project]\Project Administrators">
<LISTITEM value="Restricted"/>
</PROHIBITEDVALUES>
</FIELD>
Where XX is the Iteration ID of the root level node. Change Iteration to Area for Area Path field. So if you want to prevent users specifying just root node as AreaPath or IterationPath, put the project name (root) there and users will get an error message that "Iteration Path - Validation has invalid value.
Note: This will work for Area Path as well.