The on/off switch is activated by pressure when you rotate the food drum. It gets rusty and sticky and stays down (on) when you rotate it "off" - so you have to unplug it to get it to stop.
If you can clean it (requires disassembly, 2 small screws- don't drop them! they are hard to find.) you may find that it works ok again. If rusty, or spring is sprung, you may need to replace it.
lewstanley@hotmail.com lewis stanley, February 2007