If you really want to fuck someone put bitlocker on a drive of the ones computer. Make a usb part of the key and just hide the usb somewhere or keep it. If they ask you for help you can just plug the usb in.
Instant gratitude from someone who hates you or you can just get them fired for not meeting a deadline.
Be even more of a dick about it, just start the encryption on the whole drive while they still need to use the computer and leave them to suffer with it.