Assuming that you're trying to stop the engine from rotating while removing the crankshaft pullet bolt, right?
The easiest method is to use an impact wrench. At times, certain vehicles simply don't have the room to accommodate an impact wrench without removing the radiator or, even worse, vital air conditioning parts. Other methods are possible like wedging a screwdriver into the flywheel via the inspection plate cover which usually requires the help of a friend.
I've used a method that is cheap, that works, is safe for the engine, and can be done by yourself. Purchase 2 or 3 feet of nylon rope that is smaller in diameter than your spark plugs. Remove number one spark plug. Rotate crank pulley until it is about 90 degrees ABOVE top dead center. Insert rope into number one cylinder via the spark plug hole. Be sure to put enough rope in to fill a portion of the cylinder. Turn crank pulley counterclockwise with socket wrench. If there is enough rope inside, then the engine will cease to rotate allowing you to give enough force to break that crank pulley bolt loose.
To tighten the bolt according to torque specs after job is finished, rotate crank to about 90 degrees BELOW top dead center, insert rope, turn crank clockwise until rope stops engine, then apply proper torque to bolt.
This method is very useful when an impact wrench is not available or will not fit in the area.