It's important to remember the junior level are designed to develop the players into the best kind of hockey players they can become. They are being trained to be professionals, to be competitive, and to be classy.
You have to weigh in professionalism vs. class. Would it be "classy" for a team to stop trying as hard as they can, while they are running up the score? In a way, yes. However, it's also classless if they stop treating the game like a competitive game, and to stop treating the competition/opponents, as professionals.
You also have to bear in mind that people paid to see the game. They may not have paid to see a blowout, but I'd much rather see one team try than to see both teams just throw the puck around, not trying.
And as a coach, you reward your players who have played well by giving them at least equal playing time as the rest of the team.
No excuse for holding back in a blowout. Sure, it's the nice thing, but the reasons to play your best far outweigh the reason to hold back.