They're just one of the parasol mushrooms, probably 'Parasola plicatilis', link here http://urbanmushrooms.com/index.php?id=12. Mushrooms and toadstools are just the fruiting bodies of extensive mycelium underground, usually feeding off woody materials in the soil. There is nothing you can do to stop them growing, because the mycelium will still be there - they are not harmful to plants or your grass, but best not to eat them. They will disappear once their 'fruiting' season is over, but if you don't like the look of them, pull them off, putting them in a plastic bag, but this type of mushroom is very fragile and may just fall apart. Mycelium is present in almost all soils and most types are beneficial to the soil and surrounding plants.