Originally Posted by d_fisher
I don't believe you can block the inbound messages if someone configured their BIS account. You can however force all outbound messages to pass through the BES server by setting the Allow Other Email Services policy rule to FALSE.
What you say was all I was able to find when exploring the issue in my environment, but the real reason for my comment is to thank you for those awesome video sig files that you deploy, d_. This current one's a real kicker.