The good folks over at Microsoft Research have written a cool add-in for Microsoft Outlook called NoReplyAll that can prevent someone from replying to all or forwarding to all, on an email that originated from you.
NoReplyAll Outlook Add-In
The primary function of NoReplyAll Outlook Add-In is to add a couple of buttons to the Outlook ribbon to prevent people from doing a reply-all to your message, or forwarding it.
The add-in uses a facility built into Outlook and Exchange that is more lightweight than information-rights management but is not exposed in the existing UI.
It also includes a check for email goofs such as omitting attachments or subject lines.
This works with Microsoft Outlook, as long as you’re using an Exchange account. It however only works on English Windows & Outlook currently.
Add No Reply All and No Forward buttons to Outlook
When you install this add-in, you will see a couple of extra buttons at the end of the ribbon: No Reply All and No Forward.
As the names suggest, clicking on these will prevent recipients of your emails from performing those two actions; clicking again toggles the relevant option off again.
You can download it from Microsoft.
To install it, unzip to some directory on your Windows PC and run setup.exe.