Preheat oven to 400°F. Spray large shallow baking pan with cooking spray; set aside. Combine meatball pieces, spinach, 1/2 cup tomato sauce, mozzarella cheese and red pepper flakes in medium bowl.
Unroll pizza dough on baking pan; press into 14x10-inch rectangle. Cut dough into four 7x5-inch rectangles. Place equal amounts of meat mixture on half of each piece of dough. Fold dough diagonally over meat mixture to enclose filling; pinch edges together with fork to seal.
Sprinkle each evenly with Parmesan cheese. Bake 15 minutes or until golden brown. Meanwhile, place remaining sauce in microwave-safe bowl. Microwave on HIGH 1 minute or until hot, stirring once halfway. Serve calzones with sauce.