How to remove the possibility to send messages to staged users

There are many reasons why a Discourse instance will have staged users. In our case, we are mirroring a mailing list. Staged users aren’t (or shouldn’t be) real users, and it is rather surprising that the interface allows actual users to send them a private message. This is confusing because

a) The sender will have an expectation that their message will reach its destination.
b) The recipient either doesn’t receive any message or, if they do, they will be confused by it (who sends me a message from where?)

I’d say it is a bug but maybe there are reasons to have this functionality. In any case, is there a way to remove this feature?