title: Restrict editing in RichTextBox
body: <p>I was just wondering if there is a way that we disable editing of a specific word in the System.Window.Controls.RichTextBox? For example, I have "Hello This is a test", and I want to disable users to a edit test(which can be anything). I want to them to change whatever they want, but when they want to click or do anything on "test", it's somehow in a different color like gray(which is easy to do), but not editable. </p>
  
  <p>thanks, </p>