Don't touch them until July. Then whack them or pull them. You want a flower but not a seed head. If you top them earlier they will come back stronger. If you pull them earlier you will leave roots in the ground. The reason for topping in July is that the plant has put all it's energy into the flower and if you lop that off, it will probably die. It may take a good few years to clear a field but it is possible. Spraying is just going to kill your soil, whatever Monsanto tells you.