Is there a way to block a specific person from getting photo matches? Creepy ex, difficult guest, that kind of situation.

Yes — Snaptide thought of this. Under the album's Access Control there's a blocking feature where you can add a phone number or email address, and anyone on that list will not receive matches for that album, full stop. You can review and remove blocked contacts whenever you want. It's a genuine safety valve for real-world situations: if someone at an event shouldn't be pulling photos of themselves (or shouldn't be able to claim access), you can quietly close that door without touching anyone else's experience.