They will eventually run out of fuel. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
Just kidding obviously. Really you just yank the coil wire off. Oops, that won't work either. How about grab the fan and stop it? /forums/images/graemlins/confused.gif
All joking aside, it is a good question. Other than being able to stall the engine, as mentioned earlier either fuel or air deprivation would be your only options. Older diesel tractors all used to have the "little red knob" to pull instead of the now, key activated, solenoid that does the same job now.
Again, good question especially with a hydro, since it would be more difficult to stall a hydro especially if you didn't have a loader or other attachment to aid in the "stalling".