To broadcast a shutdown message
net send /users "message"
For example:
net send /users "SQL Server is going down in 20 minutes.
Disconnect within 15 minutes."
Note The shutdown message can be broadcast only if Microsoft® SQL Server™ is running on Microsoft Windows NT®. The users option specifies that the message be sent to all users connected to the server. For descriptions of other net send options, see the Windows NT documentation.