Private messaging should be disabled, but the item to disable it seems to have disappeared in the settings. Could potentially be a bug with the Q2A software.
Our authentication code (single signon with Project-GC) doesn't support the functions needed for the private messaging, which has been an active decision in itself.
In a perfect world we would hide the sub menu that isn't working. But it's hardly worth the priority, hopefully it will get fixed with a Q2A upgrade.