Well... I would get on his PC and upload a fuck-lot of pr0n... if he wasn't sure to get fire yet.... then he'll definitely be after that one!

Maybe send some emails from his mailbox, to say, the owner, etc?

Try this one too:

[1] Turn his PC OFF
[2] Open bottle of super-glue and empty into heat-sink/fan
[3] Close case and put back PC... walk away

... ok so I'm a nerd, you wanna fight about it?