I was taught to feed shop rags into the air intake of a runaway diesel to stop it. I have never seen one runaway and never seen one stopped so I don't know if it works. I am sure that would be my my first reaction. If no shoprags are around, your shirt, pants etc may also work.

I also cannot stess how strongly I believe in fire extinguishers. I think everyone should have them and be trained to use them.