I remeber back in the day when I was in high school. The school had a group of 386 systems no network. I modified the autoexec of the system a certain person always used. I added the following lines:
echo off
cls
echo Please insert your personal disk for verification purposes
pause
a:
dir
echo Now deleting all files
echo all files gone
echo Good By

<End insert>

Now as you can see the files where not touched but the look on his face was priceless.