title: iOS 8 UINavigationController disable Back Button
body: <p>in my Navigation Controller I need to temporarily disable the back button. I know that it can be hidden using the following or something similar: </p>
  
  <p><code>[self.navigationController.navigationItem setHidesBackButton:YES animated:YES];</code></p>
  
  <p>But that is not what I need, instead I want the back button to be greyed out and non-responsive to user touch events. Is their a way to achieve this without replacing the default back button?</p>
  
  <p>Thanks in advance!</p>