The answer to your first question is yes. You can disable local storage in the storage configuration and it will no longer show in the GUI on the left side in any of the views. Since I use local storage currently, I can't confirm that. In the worst case, you can remove all of the object types for local storage and it will never be presented in any of the drop down menus.
For your second question, I never found a way to hide the iSCSI storage when I was using it. However, I didn't really care since it didn't show up in the drop down menus as a storage choice so nobody could select it by accident (from what I remember).