The CTO of a company I worked for had a solution for this problem that, while crass, was effective.
Any time he saw an unlocked and unattended workstation he would set the home page of the browser to a hard core porn site.
Then later after the person was back at the desk he would claim to need to check on something real quick. He'd fire up the web browser, and up comes the porn site.
Then he'd pretend to be all pissed off and start yelling at the person for browsing porn at work.
Eventually, he'd explain what happened and made his point.
Any time he saw an unlocked and unattended workstation he would set the home page of the browser to a hard core porn site.
Then later after the person was back at the desk he would claim to need to check on something real quick. He'd fire up the web browser, and up comes the porn site.
Then he'd pretend to be all pissed off and start yelling at the person for browsing porn at work.
Eventually, he'd explain what happened and made his point.