Allow users to insert read only document elements.

Resolution: 
The Read Only Ranges and Document Protection feature is added in the new version of the control. Please use the RadRichTextEditor instead the RadRichTextBox. 
Please refer to our documentation for more information on the following link: http://www.telerik.com/help/winforms/richtexteditor-features-read-only-ranges.html and http://www.telerik.com/help/winforms/richtexteditor-features-document-protection.html