No, blocking someone is not possible. Understandable from LinkedIn’s point of view: networking, being able to view the persons in your networks, etc. However, you could limit your public profile. Your public profile is what the people who are not in your network will see (i.e. someone who found your profile through a Google search or LinkedIn search without being in your network). But then again, why would you limit your profile? Visitors are new opportunities for you.