Blocking another user's Facebook account ensures that she will no longer see any of your profile information or be allowed to communicate with you via the site. The process is not permanent, and you can easily unblock a person so that you can see each other on the site again. Note that unblocking someone does not restore a previous friendship – you must ask to be her friend again if you want full access to her profile. Specific applications may also be blocked and unblocked.
Go to the "Account" link at the top of any Facebook screen and click "Privacy Settings" from the menu that appears.
Scroll down to the "Block Lists" group and click the "Edit your lists" link.
Click the "Unblock" link next to a user's name in the "Block users" section to remove her account from your block list. Scroll down to the "Blocked apps" section and click "Unblock" next to the name of an application to remove it from your blocked apps.