If users are accessing accross a network it's possible to use the network message application.. (netsend??)
We used to use this where I worked a few years ago through VB and Excel.
If you can get the user name by code, you can then send them a network message. I know it's possible to get the current user (application.currentuser), but that would be you. Whether it's possible to get a list of connected (to the DB) users I don't know.
I'll try and dig out the code to automate the message sending if you want? (Though I've got to say, it may be too old to work now as it is..)
Hope this helps,
Shep