I saw a question on this recently and thought it would be good to contribute to the FAQ that Gregg started:
Incidentally, this solution is from Gregg too 🙂
<FIELD name="Iteration Path - Validation" refname="MyFields.IterationPathValidation" type="String">
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.