I would definitely not use mouthwash. (although we spray our screen doors with Listerine sometimes to keep out no-see-ums)
Sometimes I use a very mild peroxide solution (1tsp. drugstore peroxide to a cup of water) to douse the soil as we have some bacterial blights here that kill my tomatoes and basil. But really, it's futile to think you can grow plants in "sterile" soil if they're outside. It's natural for there to be fungi in/on the soil, and they are beneficial for the most part.
So, what I'm saying is I wouldn't worry about it. Just remove them if you see them growing on the surface, because if they mature they will spread spores. But they aren't doing any harm.
