No, employees cannot unschedule themselves once a shift has been assigned. A shift is fixed the moment the employee has taken a shift by clicking on "I TAKE" on the Take a Shift page or if the employee has been scheduled by the personnel planner.
When the employee wants to adjust their availability on a day they are scheduled on, a message will appear "You are already assigned for one or more shifts outside your availability. Please contact the personnel planner to resolve your availability."
The employee must contact his manager to remove their name from a shift. In this way, the manager is kept up to date.