Here's my simple solution:
I have a back end database and a front end application which is located on each user's PC.
I have a table called Admin in the back end, with a field called AdminOn, which is a Yes/No data type
The front end has the Admin table linked.
In my startup form I have code in the OnOpen event that checks the value of AdminOn in the Admin table. If its Yes, then the user gets a msgbox telling them the database is in Admin Mode, please try later.
In my mainform (which is always open even when other forms open), I add code in the OnTimer event (set for a few minutes which I'm willing to wait for). Every few minutes the form's OnTime event is triggered and code similar to that in my startup form. Except that the message says something like, "Sorry for any inconvenience, but Admin requires full control of this application and must log all users off the system. The application will now close. Please try again later."
Now obviously, when the startup code is run, it does access the back end data to look in the Admin table - but if you handle the error that is raised when a user cannot get access because Admin has it open exclusively (preferably a GoTo type handler so you can give the user a message that the database isn't available and who to contact), then it works whether or not the user can actually open the back end.