Can I show photos but stop downloads entirely? Some clients pay for exclusivity.

Yes — set the album to "do not allow guest downloads" and guests can still view their matched photos in the gallery, they just can't save them. That's useful for previews, for exclusivity arrangements, or for the phase where you're showing a client their gallery before the final delivery. Just be aware that on phones, "viewing" a photo often means someone can screenshot it — no platform fully prevents that. Downloads off is a policy statement more than a cryptographic guarantee.