Php :: Reset RDBMS Value When Browser Crashes?

I need a nudge in the right direction for a theory of a solution to the below problem.

Desired Result:

Current existing structure: FLEX RIA that communicates with MySQl DB via PHP.

We basically, have a RIA that is part of a software solution we provide to our customers, we want to restrict login sessions to one/username, which we did successfully by setting a value in our MySQL DB...the point of this was to restrict the use of username(s) to one application access point and create the ability for us to charge for additional usernames, if so desired by our customer.

Problem:

Although, we successfully restricted user logins to one session, we ran into a problem when the RIA connection with the DB was inadequately terminated (eg., browser crash, OS crash, flash player error, etc). When these crashes happenned the value that was set in the DB for the user, showing them logged in, would persist and thusly lock them out of our software application. We would have to go into the database and manually reset their logged in status.