Some people may think that microwaving a sponge kills its tiny residents, but they are only partly right. It may nuke the weak ones, but the strongest, smelliest and potentially pathogenic bacteria will survive.
Then, they will reproduce and occupy the vacant real estate of the dead. And your sponge will just be stinkier and nastier and you may come to regret having not just tossed it.
Other cleaning methods (running it through the dishwasher or washing machine, dunking it in vinegar or other cleansing solutions, even cooking it in a pot) also don’t work. It’s not just that they don’t work, they make matters worse.