To make the RadRichTextBox read only, you have to set its IsReadOnly property to True.
Read only mode
radRichTextBox1.IsReadOnly = true;
RadRichTextBox1.IsReadOnly = True
Except the ReadOnly property, there are several other properties that can be used to control the response of the RadRichTextBox towards the user actions against it:
Enabled - setting this property to False will disable the entire control. The user won't be able to enter any input in it and to scroll the contents.
IsSelectionEnabled - setting this property to False will disable the user to perform any selection inside the RadRichTextBox control.