Try using a oven cleaner or better yet, Sam's sell a Oven, Grill and Fryer Cleaner that works great. Spray either on and wait a few minutes then if you have a pressure washer, use it to rinse. This should work. You may end up having to pressure wash the rest of the patio area or whatever it is. Lol... It'll make the whole area look new again.
If this doesn't work, time and sunlight will do it's natural best and bleach it away.