Sprinkle fresh spills inside the oven with salt. The salt will absorb the spills and can be brushed away once the oven has cooled.
Do this only if your kitchen is well ventilated. Preheat the oven to 200F and then turn it off. Pour 2 cups of straight ammonia into a non-aluminum pan and place on the middle rack. Shut the oven door and let sit overnight. By morning the grease will be easy to wipe up.