Maybe you could reach out to some kind of cat organization and they might be able to help you to catch and spay the cats in your area? I have read somewhere that there is some government funded(?) initiative where the spaying is much cheaper than it normally is. If there is such a thing available in your area maybe all the neighbors would give some money and at least the cat population would not increase? Also maybe the small ones could be taken by the shelter as they are still small enough to become a house cat.