You can delete existing friends at any time by following these steps:
In the Friend area of your home page, click the Edit Friends link in the top-right corner (Figure 4.13).
On the Edit Friends page, check the box beside the photo of the friend you want to delete.
Click Delete Selected to delete that friend.
Friends whom you delete will not be notified about the deletion, though they will probably figure it out when you no longer appear as their friend.