You have to pay full salary regardless of whatever you deem work is and however you judge the status to be done or not. You can ask the employee to stay longer do it enough and that employee now pushes off their night and starts coming in later to.. because they are going to be staying late anyways. And nothing gets done after 5pm aside from making plans to hit the bar at 7/8/9. The CEO notices this and this employee has a rep of staying late. Soon he is your boss.

What would you rather have again? An employee finishing early sounds good to me