Here is a nifty add-in for Outlook that prevents people from replying or forwarding to all on an email sent from you. In a nutshell, the add-in adds buttons to the Outlook ribbon that prevents someone from doing a reply all on email sent from you or forwarding it.
Once you install the add-in, you’ll see two new buttons on the ribbon: 1) No Reply All and 2) No Forward. If you click either of these buttons, recipients on your email are unable to perform the respective action.
Visit Microsoft Research to download the add-in.