I swear to God, it's been so long I have no idea.
Firstly, pity them. Then explain to them that we live in a never-ending universe and to imagine that this small planet is the only one with life, among trillions of other planets, is ignorant. If they are weaker than you, then say, "Try reading a book". If they are stronger than you, run.
Scientists have been working on this question for almost 100 years and the best answer they have come up with is that we are stupid.
I keep an open mind. Some things I firmly believe in, like alien life and a deeper truth behind pyramids. I wanted a safe space shop for those who believe in all kinds of phenomena and one of my favorite things in the whole world, aside from puppies, is talking to my customers about their experiences and beliefs.