I've tried everything I can think of to kill a running notebook that is stuck in a loop and nothing works. The Interrupt Kernel button is always grayed out, tried restarting the kernel through the command prompt (P Key) but this doesnt seem to do anything either. The only way I've figured out how to stop the process is to kill the entire application in task manager.
In the task manager, can you see and kill the python process specifically? Still not a pretty solution, but perhaps better than killing the entire application.
Chris, did you ever figure this issue out? I've got similar behavior. My interrupt kernel button is always greyed out when the kernel is busy. I've tried using the keyboard shortcut (I, I) as Dan recommended, and this does get rid of the asterisk next to a cell denoting that it's running, but the kernel itself doesn't stop, meaning I still can't run any other cells.