crickety not all want a private message sent to them
Not all want email spam sent to them either, but it's nearly impossible to stop, other than changing email addresses.
I'd suggest treating your PM folder like a spam folder, and just ignoring it's existence? If it's a single user who abuses the feature, perhaps functionality to block specific users instead of turning off messaging completely?
crickety Would it be possible to on/off the Send Private Message button?
Turning on/off the "Send Private Message" button would prevent you from sending messages to other people. But is that what you're requesting? Or are you requesting functionality to prevent messages from being sent to you (i.e. others "Respecting [your] personal privacy"), like auto-deleting messages that are addressed to you?