There were a few kids at my school who played with the "Net send" command to drive people nuts. In a similarly childish retaliatory move, I made a batch file to cause them a few troubles when they tried it. It runs only on Windows, of course.
Whenever somebody tries to issue a "net send" command, this batch file will instead cause the message to repeatedly send back to the originating computer in a loop. It's completely transparent otherwise and will not give any other clue that something is wrong with "net send."
To use this, copy "net.bat" to whatever folder the command prompt starts in. Good luck explaining stuff to your sysadmins, because I take no responsibility for the consequences.