Preheat waffle maker. Spray with cooking spray. Slice polenta into slices, about 1/4 inch thick. Place polenta on waffle iron and close lid. Press down firmly to squish them a little bit. Let cook about 2 minutes, then press down again. Let cook until browned and crispy, about 5 minutes. Remove from waffle iron and set aside. Keep warm. Repeat with remaining polenta.