My office phone looks like it can open using a flathead screwdriver. It has no screws, but there's a seem that runs along the side.This isn't 1986 and it is hard to put coins in a phone.... plus with new phones you would probably on be able to fit $1.68 in change in one.
Go to Home Depot to buy a remotely controlled relay and a cheap octopus plug. Unplug his computer from the wall and plug it into the octopus plug. On the same octopus plug, connect the remotely controlled relay so it forms a short circuit. Wait until he's doing something on an important document then remotely short the circuit his computer is on so it trips the breaker and his computer turns off.
warning: you might be fired because this is an obvious fire hazard