Jeroen,
I cannot advocate anything that would do a forced logoff/shutdown at risk of lost data. On my LAN, we run reports of logon/logoff stats and then 'educate' the users. Sometimes, we call the user at their home to come back in to work to save their file and logoff gracefully.Also, we have 'tattler' software on internet activity, the side effect of which, is that wiley users will find an unsecured PC to do their porn surfing. They (we 
) are a malicious bunch, and if they encounter an unsecured email, will use it to disparage management. Of course, the unsecured user will take the heat, for which (s)he has no defense as company policy is very clear on securing the PC.
Once, on a contract job I was on, with the collaboration of management we sent off a letter of resignation from an unsecured PC. When the unsuspecting user returned, he read the repy from management, accepting his resignation with best wishes in his new endeavour. He went with hat in hand to the manager to plead for his job back. This became common knowledge and changed most user's habits.
Ashpoint,
Perhaps after the toothless management receive a few disparaging emails, they may better enforce policy.
