Theres still a chance one of them might pick up the book and if its spoiled for them then that chance drops.
Also I was a relatively quiet student but if this were put in place it would give me every reason to make as much noise as possible so I couldnt hear the teacher spoiling things and never look at his board. Possibly steal all his chalk/marker.
No theyre likely to remember how they lived through the experience, minimise the memory pf what they felt at the time and then take a lax approach to spoilers. Thats why people who are bullied become bullies, that's why people with smoking parents are more likely to smoke.Secondly, a punishment isn't a punishment unless it's something the kids don't want to have happen. And seeing as they'll likely remember having the entire third season spoiled for them, they're not likely to do it to others for no reason.
Then you'll only have yourself to blame when your children grow up with bad taste in tv shows. Because when something good comes along you decide to ruin it. Enjoy listening to them talk about Jersey ShoreIn fact, I might do it as a parent.