As follows a joke you can make to a colleague who goes away from his desktop and leave the pc unlocked.
But you can think about something more annoying...
Why don't you alterate the site he's currently browsing?
At instance you can make a button or every button unclickable... in this way!
When your mouse is over the selected button, it moves randomly so you cannot click it :)
Very annoing, isn't?
You can also try it here: http://jsfiddle.net/QqanL
Have a funny day ;)