Windows Applications and Putting Our Foot Down

We have inherited the chore of implementing a fairly poorly designed software package as part of a large scale deployment to manage data for about 88,000 - 90,000 people.

Approximately 1,000 users may be logged in at any given time to manage this massive amount of data.

We are using (sadly) a Client/Server architecture with SQL Server as backend and VB (obviously) as front end. I would like to move this to N-tier over the long term, but we are stuck, in that respect for now.

My question is as follows:

In what way might I manage user connections to the database and disallow log-in at the discretion of network admins. I thought of a switch to disable log-in, but that would not log-out folks who have logged in when the system was "Available". Any ideas?

Could you do something like when the admins turn off someones access, you send out a message (Probably TCP/IP message) to all the machines that says X id no longer has access. If it is their machine the application will stop functioning and take them to a login screen (or something).

You could also have a timer running on the VB applications that check for a database table that checks if their login still has access (or lost access), though that would require constant checking and would slow things down.

Just a couple of thoughts I had.

Michael