Screensaver, wait 5 minutes - On Resume Password Protect
That will protect you from most of the Goatboys.
Why stop pranking because the user "merely" locked their workstation.
If the fool left a bootable drive on their machine you can download bootdisks and BIOS password changers and go to town. Change everything you want.
You could re-arrange all the keys on their keyboard to spell swear words on them.
Mess around all of their monitor display settings making sure you set the display language to Swahili or something, good luck in getting that back to normal.
Flip the 110/220 switch on the back of the power supply to 220v.
And top it off with the old shoe polish on the headphones trick.
Amateurs... Touch my PC and feel my wrath.