I think I'd just pop the F off his keyboard and see how long it takes for him to get himself understood.
Day 1 (Monday)...
Dude: "My f'n F is f'n gone, mother f'er!"
Boss: "What?"
Dude: "It's totally f'd up, my F is gone"
Boss: "You're not making any sence"
Dude: "Listen, some f'r is f'n with me and took the F off my f'n keyboard"
Boss: "You should use more of your sick time."
Dude: now shouting to the entire floor "Which ever f'r took my F, I'm gonna f'you up good. You're all on f'n notice!"
Day 2...
Now you replace the F, but remove the U.
Day 3...
Replace the U, remove the C.
Day 4...
Replace the C, remove the K.
Day 5...
Remove every remaining key except U.
Saturday & Sunday...
Tell everyone you know about the crazy guy who quit on Friday without giving two week notice.