It is possible though cumbersome.
- In the new email you're composing, navigate to the Developer tab (Outlook hides this be default so if you don't see it, right click on the Ribbon, select Customize Ribbon and check the box next to Developer).
- Select Design this Form
- Select (Actions)
- Double Click on Reply All
- Uncheck the box that says Enabled
- Click Okay
- Hit Save
You can navigate back to your message my selecting the Message tab, but I generally hit save and close the email and then go back to in my drafts folder and reopen the email to make any edits before sending. This can be done before you send or after you send.
Hope this helps!