If you have McAfee Total Protection installed on the machine you can use the Parental Controls tools to block Facebook.com altogether - that way your teenager would not be able to access any accounts on facebook. If you have multiple children that are using the computer you can set up individual accounts and set the protection how you see fit for each account. You can block web sites by category (i.e.: Adult,gambling, etc) by age group or specify site that you want blocked. If you do not have Total Protection you can use McAfee Family Protection which is a stand alone parental control product.
Hope that helps,