How do I block someone from contacting me?

If someone is bothering you, the easiest and least aggravating way of dealing with it (other than somehow making peace, of course) is blocking that person.

Go to his profile page.
Scroll to the bottom, and click on the "Block this member" button.
Confirm your choice by clicking on "Yes I'm sure" next.

You can unblock the same way.
Blocking has a number of consequences:

You cannot see the profile of members whom you have blocked, nor can they see yours.
You cannot send them messages, nor can they message you.
You cannot write recommendations for them.
The only exception is abuse flags. The reason is so that abuse flags can not be evaded by blocking.