As I understand it, verbally threatening an individual with physical assault is a crime. It is in fact, assault, distinguished from assault and battery, which means actually physically striking someone. If anyone threatens you physically online, you have the legal right to sue them for it. You also have the right to ask the police to arrest that person, although you probably won’t have much luck persuading them to do it. Nevertheless, this is a principle in English common law that dates back hundreds of years. More people should call on this principal in the cyber age. It would probably greatly improve the courtesy level online