OK, here are two more suggestions:
1. This is a compilation of the others, but it works as a chain reaction. First, reverse his entire office ala The Bizarro Jerry. Then, fill his computer keyboard is marijuana plants (these may be difficult to obtain, but they will pay off later). Go into his computer, delete all of the icons on his screen, and change his wallpaper to this:
http://whiteafrican.com/wp-content/blue-screen-of-death_1152.jpg Then, put his mouse inside a block of jello.
Now, after he figures out where his computer is, he'll turn on the monitor and see what has happened to his computer, thinking that it's the blue screen of death. He will then get upset, possibly yelling, and hopefully someone in authority will come in and notice the marijuana plants. He will then either be fired, or hauled off to prison. During that period, you may eat the jello and laugh, possibly shouting out, "AIR HORN ME, WILL YA???" as he's brought outside.
2. Buy a large bag of rubber bands, and use them as a form of
Chinese water torture. While in a hidden area, shoot one at a time at his head until he slowly goes insane. When he gets up and finds where you are, take an air horn and blast it right in his face. That's good revenge.