Funny :) I had made something similar on my fellows in the computer pool. I used msg USERNAME /server:COMPUTERNAME "Your message here" (or similar..).

So there was this girl working on something I send a msg to her IP "We discovered that you used up your usage time quota. We will start to clean up your space in 2 mins". She got that displayed, was wondering and just clicked "ok" continuing the work. Past 1 min, I send her another message "the clean up will start in 1 min..". Again she clicked it away. After further one minute: "the clean up starts now. 3% finished." .. she slowly got uncomfortable, but clicked away again. So.. few seconds later I send her "13%.."

She got really uncomfortable. Saved her work. Shutdown windows and went to another computer. Just as it booted up and she opened again her files: "20%..."

She got very pissed. Closed her files and left the computer pool wildly gesticulating and angry on the computer department. I did not see whether she went straight to the admins. It was funny. It was easily discoverable because each message was displaying the info of the sender. so she just needed to see it was sent from the computer behind her :)