This Windows application blocks a computer from social media, addicting websites, games and even programs. It is a perfect solution for businesses and schools worried that their company computers are being used for non work/school related activity.
You can also schedule the blocking. Maybe open it up during lunch hour or after school?
An administration password is required to configure and disable the program, so users can't shut it down, even from the task manager.
Cost: $10 (½ of that is donated to charity).