Use new access denied extension point.

lib/controllers/backend/spree/admin/admin_controller_decorator.rb

@@ -1,26 +1,11 @@
 Spree::Admin::BaseController.class_eval do
-
-  # Redirect as appropriate when an access request fails.  The default action is to redirect to the login screen.
-  # Override this method in your controllers if you want to have special behavior in case the user is not authorized
-  # to access the requested action.  For example, a popup window might simply close itself.
-  def unauthorized
-    if try_spree_current_user
-      flash[:error] = Spree.t(:authorization_failure)
-      redirect_to spree.admin_unauthorized_path
-    else
-      store_location
-      redirect_to spree.admin_login_path
-    end
-  end
-
   protected
 
-    def model_class
-      const_name = controller_name.classify
-      if Spree.const_defined?(const_name, false)
-        return "Spree::#{const_name}".constantize
-      end
-      nil
+  def model_class
+    const_name = controller_name.classify
+    if Spree.const_defined?(const_name, false)
+      return "Spree::#{const_name}".constantize
     end
-
+    nil
+  end
 end

lib/spree/auth/engine.rb

@@ -44,6 +44,26 @@ def self.activate
           end
         end
         ApplicationController.send :include, Spree::AuthenticationHelpers
+
+        Spree::BaseController.unauthorized_redirect = -> do
+          if try_spree_current_user
+            flash[:error] = Spree.t(:authorization_failure)
+            redirect_to spree.unauthorized_path
+          else
+            store_location
+            redirect_to spree.login_path
+          end
+        end
+
+        Spree::Admin::BaseController.unauthorized_redirect = -> do
+          if try_spree_current_user
+            flash[:error] = Spree.t(:authorization_failure)
+            redirect_to spree.admin_unauthorized_path
+          else
+            store_location
+            redirect_to spree.admin_login_path
+          end
+        end
       end
 
       def self.backend_available?