If you have a spare carbon foam on hand for when you need carbon, just rinse out the carbon foam as if it was a plain one and it should be fine. Exhausted carbon, in a foam, is probably just as good as a plain foam for developing the bacteria that your filter needs to properly process ammonia. The main reason we don't toss old sponge has little to do with cost and much more to do with fish health. You need the bacteria in the used filter media, you don't want to throw that away and threaten the health of your fish by doing so.
