Andreas,
I can see how that would be useful, but unfortunately, you would have no way of knowing if the user was legitemately on a long call or not and, therefore, you would risk cutting off a real, live call.
Like Mark said, you could do something with a Handler, but I would be more inclined to run a report and see if it's actually happening. If so, deal with the individual user(s) on a case-by-case basis. You could even set an alert up to let you know when it ocurrs and then have a Supervisor go in, listen in on the call (to see if it's active) and manually disconnect it if not.
Just my 10 cents...