The problem is that if you block the user, they just come back with another user ID and it becomes a war of attrition as to who gives up first. The best idea is simply no to use social mediia like Facebook, or if you do, then make yourself anonymous.
Or else make you profile private, and be damn careful in your choice of "friends"
However the bottom line is that if you are vain enough to post your details on facebook, then you are exposing yourself to every nutter in existence