There's an easy solution: only give UBI to full citizens. Since you usually need to live in and pay taxes in a country for ~10 years to get full citizenship, this is an easy solution.

Like most easy solutions, it's probably too extreme, some permanent residents definitely deserve UBI too. But it doesn't seem difficult to imagine an appropriate test based on years of residency, et cetera. It's certainly an easier problem than some others, like how to pay for UBI.