There are a couple of different ways to accomplish this:
1) Use a different form for the reply and make all the controls on that form's compose and read layouts read-only.
2) Use a custom property to store information about the "state" of the message (e.g. unsent original, received original, unsent reply, received reply) and use code in the Item_Open event handler to disable/enable controls depending on the state. You probably would set the "state" property in the Item_Send event handler.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers