If you don't plan on meeting him another time and don't really know anything about him to begin with, it shouldn't be too hard. It's not like you bonded with each other or texted for a while etc.
I'd just ghost him. It isn't bad karma, while he may be hurt in the moment, he'll come to realize later that there's more to life than boys & you'll have done him a huge favour. That's how I justify it.