Usually when I wanted someone out, I put an OnTimer event on the form with an interval of something like 30 minutes. When the timer fired, I had it put up a modal dialog box to warn the user that his 30 minutes had expired. Then I reloaded the timer for 10 minutes and did it again. Then 5 minutes. Then every minute. But I never actually killed the form because I did not want the user to lose data. Using the modal dialog meant the users HAD to click the OK in order to proceed, but they COULD proceed. It was just that once the time dropped down to once per minute, it got annoying enough that they would finally take the hint.
As Minty points out, you CAN use the OnTimer event to do things, but be careful of doing something that could leave your database in an inconsistent state. That, in my book, is worse than a user overstaying his/her welcome.
If you DO try a timer event, remember that the timer ticks in MILLISECONDS, not seconds, so 30 minutes is 1,800,000 milliseconds. The timer is good for 32-bit (LONG) integers, so you can specify that number. But also remember that drastic control can result in drastic damage.