Protecting victims has to become the first priority. Reacting to a crime that's been committed rather than trying to prevent one does little to help a terrified victim. Putting someone's life in fear from verbal threats or overt actions should be a crime itself. Victims need protection from their stalkers whether the person is incarcerated or not. The legal system and law enforcement can't be a savior for everyone's problems.
Individuals are the ones to best protect themselves.
Letting someone down easy only prolongs the rejection process. The earlier the better for you and the pursuer. No means no. Be concise, diplomatic and firm, but allow the individual to maintain his dignity. You don't have to be harsh or rude.