How to stop people from replying to automated messages?

[409industries]

This is a somewhat small problem, but something that is very irritating and I am sure can be easily fixed. This is regarding PM notifications via email:

On Sun, Feb 7, 2010 at 10:37 PM, XYZ Forums <address@domain.com> wrote:

DO NOT REPLY TO THIS EMAIL!
***************************
Dear userA,

You have received a new private message at DirtyImpreza Forums from userB, entitled "Re: blah blah blah

To read the original version, respond to, or delete this message, you must log in here:
http://www.domain.com/forums/private.php

This is the message that was sent:
***************

People are replying to the email notifications of there being new PM's in their inbox. I want to make sure that if they reply to these emails it gets bounced back to them. I have a particular email address configured to the sender of these notifications and im constantly getting random replies to the address because its actually a valid address on my domain.

What do i need to change so that the notifications still get delivered via email, but make it so that people can't reply to them?