I think the first step should be reasonable discourse with the bullies parents, not threatening them. But I also think that threatening people can be a good solution. In the past I was involved in a situation where the law just couldn't really do anything until it was probably too late. A simple quick threat later, and the problem went away. I'm not saying I'm proud of it, or that I prefer it as a solution, but we're still animals, and we respond to threats.