Too Many Aliens

I've never gone to an Alien Night at a bar. They're just everywhere. They especially seem to show up at festivals in San Myshuno a lot, but are also wandering the streets as well. There's just too many and I want to find a way to stop them spawning in the world and the "Not in World" household pool. They're taking up about 20% of my Unplayed Households and I really want to get rid of them.