That is not exactly clear what you try to achieve.
Can you use two richboxes, one as read-only box just to show the "constant" content and another one nearby / below with the editable content?
As soon as the user is ready, you simple merge the content of both controls.
Would this simply trick solve the problem?
[EDITED]
Please take a look at this solution:
String protection in RichTextBox .
Suggestion:
Why don't you use a control inside the RichText box? you can insert buttons and other controls, so i guess inserting a Label or a TextBlock might do what you require.
Answer:
I tried to put some controls in RichTextBox and added some code to protect controls from being deleted, and it worked.