It is a good practice to start what we call "Hasslehoffing"

It is where you change the background to a picture of David Hasslehoff every time someone leaves their PC unlocked for a long enough time to change the background. The more it happens, the sexier he gets.

I urge other colleagues to do the same. The only defense there is against that is to lock your PC every time you leave your desk. It really works.