I have a "system" user that sends "Welcome to the website" private messages as well as announcements to users. I am able to send these messages via rules; however, I don't want users to be able to reply to these messages.
I tried disabling private messages for the system user but that option disables both the receiving and sending of private messages (I checked-- if private messages are disabled for a user, no private message can be mailed from that user using rules).
I don't need a solution that is bulletproof; hiding the reply form on the private message view screen is sufficient. However, I dug through the private message module code and was unable to find any way to do that.