1. A method, comprising: capturing images periodically from a camera on a computing device; comparing a captured image to another captured image to determine whether the captured images have in common an image of a user; determining that the captured images do not have in common the image of the user; locking the computing device upon determining that the captured images do not have in common the image of the user; and when each of the captured images contains images of multiple users, determining whether none of the multiple users in each of the captured images matches the stored image of an authorized user, and locking and preventing further operation of the computing device by each user of the multiple users in each of the captured images, upon determining that none of the multiple users in each of the captured images matches the stored image of the authorized user.