Upon what I understood from your question no need to create the categories (‘unclassified’ to ‘top secret’) I believe you can do the following
1- On the Lotus Domino administrator
2- Go to the configuration document of the Hub Administration server
3- Create a rule not to allow message to sent to this ” selection of allowed domains”
4- Create Declaimer message to be returned to the sender to inform him that his message was not delivered because violates the rules
if you need further assistance please let me know .