Go to your address book and click the 'Search' tab. Enter the category that contains the people you want to delete and click search. The resulting page will give you a list of contacts in that category. In the title bar, above the results, you will see a checkbox that enables you to select all the entries on the page. When you click the 'Delete Checked' button, all of those contacts will be deleted from your address book. Remember, once you delete a contact, they are permanently removed and cannot be retrieved again.