Did you know that you can complete tasks, when you close your weekly timesheet?
You do this by marking the checkbox below Compl. to the right on the weekly timesheet. If an employee completes the task too early, you can navigate to the given project and change the status for the employee in the project plan.
It is possible that one employee completes the task, while the rest of the resource group continues tracking time on it. You can turn this functionality off in the System administration under Projects -> Project module settings -> Scheduling – Let employee mark tasks as completed.