As it currently stands, there is no way to stop a user from private messaging another. This leaves it open for abuse, significantly.
By either adding the option to block users from sending private messages or disabling the option to private message a user, unwanted, harmful, or spam messages can be deflected from a user.
Some authors may not want to be directly messaged, either, whether they just don't want to be bothered or don't have the time to deal with them. There are also some authors that use different contact methods as they are easier to manage, so disabling private messages means questions can be routed to the easier platform of communication.
Additional Note: it appears there is an "Ignore Users" tab in a user's settings, however the option to ignore isn't visible. If the option to block users from private messaging another, it might be worth having an easy-to-access button to ignore the user.