On our ecourse site we’ve defaulted teachers to opted out of private messaging. However, they still receive PMs because users simply go into their inbox now and compose a message to the teachers there. I tried it myself and can confirm that as well. I’ve opted out, then sent myself a PM from a test account via inbox. I even got the notification that there is a new private message as well, which, of course, I can’t check since I am opted out.
From my perspective this should be a complete opt out so that no one can send a message regardless which method they use. At the very least there should be a message stating that the selected user opted out of private messaging and cannot receive PMs.