Well here is an idea:

Make a status table. This table holds a numeric value 0 = okay 1 = time to log off.

Then have the client applications run a check of this table on a frequent basis. If the client sees the status 1 it prompts the users to shut the application down. Or you could make it do a custom shutdown method.

Also have this table checked on startup. If the status is 1 again then don't let users login.

When you are done just set the status back to 0 and everyone can get back in.