I have a Nikon D3100 with the Energizer ENF-300N flash. However, I am running into an issue: when I go to take a photo with any of the auto modes, the internal flash still pops up and fires.
For starters, stop using scene modes. If you absolutely have to use any form of Auto mode, try using P mode instead and see if the internal flash pops up (personally, I can't stand using anything but full Manual mode when using flash).
Also, is the Energizer flash actually on? If it's off, the camera will not detect it, and will force the internal flash to come up.