Freedom of speech is fine but not threats to life or downright slander. I understand he threatened to drown Daley, which is probably laughable.
The problem is this has become a world of senseless acts of violence and I wonder how we would all feel if it was us on the receiving end. Sadly celebrity status, politicians, journalists etc., have to deal with such happenings on a regular basis but there are times when they must wonder just how serious a nutter is actually following them. Think about Jill Dando for just one example.
It is true to say the police cannot deal with the numbers of complaints but the first time they ignore a complaint and someone is harmed the press and public demand heads to roll. They can't win on this one.
How does it get stopped? It probably won't. Why, because the only thing to do is make everyone put their name to their tweet but then Twitter and the like would not be viable because it works by allowing anonymity giving the idiots and malicious bullies the chance to be the cowards they are.