Unless you've got the gearbox off good luck securing the crank from turning.
You've got a few options:
1) use a rattle gun
2) position a breaker bar so it will wedge against the chasis when the motor is turned over, crack the starter, and your bolt is undone. - you can probably wedge the bar against the ground even, depending - that's the way I've done it.
3) fill one cylinder with oil or water and put the spark plug back in. When done, take spark plug out and turn motor over by hand to blow the water out of the cylinder - oldcorollas idea, but I love it
4) wedge a large screwdriver in one of the flywheel teeth (if gearbox is off)