I like what hvert said about "sign up for a class" or some other activity that makes you unavailable. Even if you don't really sign up for something, having that reason for being unavailable for the ridiculous imposition your employer is placing on you, this will be a legitimate excuse to not work so much overtime.

Also, and I say this from my own experience, taking a class is a great morale booster, something you do for your own self-improvement.

Like you, I have always been reluctant/afraid to say "no." Your employer doesn't care about you. It's modern day slavery. You alone must take care of yourself!