When you block a user on Facebook, he cannot find you in searches or see any of your activity on the site--you become invisible to him, and he becomes invisible to you. Blocking is reversible, and once a user is unblocked, you can immediately see each other again. After you unblock a Facebook user, the site requires you to wait 48 hours before adding him back to your list of blocked users.
Go to the "Account" link in the top-right corner of your Facebook screen and choose "Privacy Settings."
Scroll to the bottom of the page and click on the "Edit Your Lists" link in the "Block Lists" section.
Enter the name or e-mail address of the user you want to block in the "Block Users" fields and click the blue "Block" button. Facebook will search for a list of users that match your entry. Click "Block" next to the correct user.
If you cannot find the correct user in the "Block List" search, go to the user's profile and click the "Report/Block This User" link in the left column of the screen.