Hope it helps..
Add the following code in functions.php of your theme
/*remove email addresses from private messages
$replacement = "[removed]";
/*if you dont want any replacement text, replace the above line with
$replacement = "";
return preg_replace("/[^@\s]*@[^@\s]*\.[^@\s]*/", $replacement, $message);
/*the code is short and works in most cases but not full proof. Check http://www.linuxjournal.com/article/9585*/
It’ll remove all email id’s from message subject and content. While replying(ajax-no postback) the message is displayed to sender unaltered, but don’t panic, it actually is removed and receiver will never see the email id.